Inclusion Criteria:  Patients with metastatic disease in the lung and/or liver from any primary malignancy considered incurable by local therapies  One prior anti-PD-1/L1 therapy allowed One prior anti-CTLA-4 therapy allowed  The target lesion(s) must be measurable as per immune-related Response Evaluation Criteria In Solid Tumors (irRECIST) and repeated measurements at the same anatomical location should be achievable  Participant must have at least 2 measurable lesions at screening  Abscopal cohort: At least one lesion will receive NBTXR3 and high dose radiation (high dose target lesion). The other lesion(s) (non-treated target lesion) will be followed for response and it will not receive NBTXR3 or radiation therapy (RT) RadScopal cohort: At least one lesion will receive NBTXR3 and high dose radiation (high dose target lesion). The other lesion(s) will only receive low dose radiation (low dose target lesion)  Amenable to undergo the image guided (endobronchial ultrasound bronchoscopy [EBUS] or computed tomography [CT] or magnetic resonance imaging [MRI]) intratumoral injection of NBTXR3, in up to two (2) high dose target lesions, as determined by the investigator or treating physician at screening  Intratumoral NBTXR3 injections only allowed in lung or liver lesions  Selected high and low dose target lesions must be amenable to receive radiation therapy as determined by the investigator or treating radiation oncologist  Allowed high dose RT regimens are 50 Gy in 4 fractions or 60 Gy in 10 fractions Allowed low dose RT for RadScopal cohort is 1.4 Gy per fraction in 4 - 5 fractions to only low dose-target lesion(s) determined by the investigator or treating physician  Patients can receive radiation therapy for symptomatic metastatic disease prior to enrollment  Untreated bone metastases allowed to be treated on this study, as low dose target lesions, with low dose RT in the RadScopal cohort, at the discretion of the investigator or treating radiation oncologist Age >= 18 years Eastern Cooperative Oncology Group (ECOG) performance status 0-2 Hemoglobin >= 9.0 g/dL Absolute neutrophil count (ANC) >= 1,500/mm^3 Platelet count >= 100,000/mm^3 Leukocytes >= 1500/mm^3 Creatinine =< 1.5 x upper limit of normal (ULN) Calculated (Calc.) creatinine clearance > 45mL/min Total bilirubin =< 2.0 mg/dL Aspartate aminotransferase (AST) / alanine aminotransferase (ALT) =< 2.5 x upper limit of normal (ULN) or =< 5 x ULN for patients with liver metastases For patients with lung metastases, adequate lung function with expiratory volume in 1 second (FEV1) >= 0.8L or >= 35% predicted and carbon monoxide diffusing capability (DLCO) >= 40% with or without bronchodilator within 28 days prior to NBTXR3 injection Patients who meet the criterion above without oxygen (O2), but need acute (started within 7 +/- 3 days) supplemental oxygen due to tumor-caused obstruction/hypoxia are eligible, provided the amount of the O2 needed has been stable Negative urine or serum pregnancy test =< 7 days prior to NBTXR3 injection in all women of childbearing potential (WOCBP). WOCBP must agree to follow instructions for method(s) of contraception for the duration the entire study period and 160 days (~5.33 months) after the last dose of anti-PD-1 treatment. Local laws and regulations may require use of alternative and/or additional contraception methods. WOCBP who are continuously not heterosexually active are exempt from contraceptive requirements but should still undergo pregnancy testing Signed informed consent form (ICF) indicating that participant understands the purpose of, and procedures required for, the study and is willing to participate in the study  Exclusion Criteria:  Prior radiation therapy received to the selected high dose target lesion(s)  Previous radiation to low dose target lesions allowed as per investigator or treating radiation oncologist discretion  Symptomatic central nervous system metastases and/or carcinomatous meningitis  Participants with previously treated brain metastases may participate if those lesions are radiologically stable (i.e., without evidence of progression for at least 4 weeks by repeat imaging at screening), clinically stable, and without requirement of steroid treatment for at least 14 days prior to NBTXR3 injection  At screening, past medical history of:  Interstitial lung disease Unresolved organizing pneumonia (i.e., bronchiolitis obliterans, cryptogenic organizing pneumonia) Any grade 4 radiation toxicity  Unresolved, radiation or immune checkpoint inhibitor (ICI) related  Pneumonitis Bronchopulmonary hemorrhage Abdominal hemorrhage  Unresolved gastrointestinal (GI) related events  Diverticulitis Colitis Intra-abdominal abscess GI obstructions Abdominal carcinomatosis Any known risk factor for bowel perforation History of severe (grade >= 3) immune-related adverse events observed with previous immunotherapy (anti-PD-1/L1 and/or anti-CTLA-4) or known sensitivity (grade >= 3) to any excipients  Has received any approved or investigational anti-neoplastic agent or immunotherapy within 4 weeks prior to NBTXR3 injection  Except anti-PD-1/L1 or anti-CTLA-4 therapy, which will not require a washout window A reduced washout window may be considered for therapies with short half-lives (i.e., kinase inhibitors) after discussion with Nanobiotix and investigator  Active autoimmune disease that has required systemic treatment in the past 2 years (i.e., with use of disease-modifying agents, corticosteroids, or immunosuppressive drugs)  Replacement therapy (i.e., thyroxine, insulin, or physiologic corticosteroid replacement [=< 10 mg prednisone] therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment  Has not recovered from adverse events (AEs) due to previous anti-neoplastic or immune-oncology therapy and/or interventions (including radiation) to =< grade 1  Participants with alopecia and =< grade 2 neuropathy may be eligible  Any live-virus vaccine used for prevention of infectious diseases administered within 4 weeks prior to NBTXR3 injection  Except killed-virus Influenza vaccine Exception of other vaccines (e.g. pneumonia) is at the discretion of the treating physician after conducting a personalized risk assessment on a case by case basis Prior allogenic stem cell transplantation or organ allograft Known contraindication to iodine-based or gadolinium-based IV contrast Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, renal failure, cardiac arrhythmia, or psychiatric illness that would limit compliance with treatment Known active, uncontrolled (high viral load) human immunodeficiency virus (HIV) or hepatitis B or hepatitis C infection Female patients who are pregnant or breastfeeding  Women of child-bearing potential and their male partners who are unwilling or unable to use an acceptable method of birth control to avoid pregnancy for the entire study period and up to 160 days (~5.33 months) for female participants, and 7 months for males participants or female partners of male participants that are of child-bearing potential, after the last dose of anti-PD-1  Acceptable methods of contraception are those that, alone or in combination, result in a failure rate of < 1% per year when used consistently and correctly Any condition for which, in the opinion of the investigator, participation would not be in the best interest of the participant (e.g., compromise the well-being) or that could prevent, limit, or confound the protocol-specified assessments