Whenever the UKVI agency decides to reject an application, they send a rejection letter to the applicant with the contact information they have provided, either via mail or email. The matter of how you receive the decision letter depends on the details provided in the application. If a postal address has been provided, UKVI reposts a handwritten version of the refusal letter through the mail. If the applicant has applied online, the UKVI may forward the refusal note through email since it would have reached the applicant faster.
The exact reasons for the decision are available in the letter. If an application was rejected, you can also find the reasons for it in the letter. The letter gives details on any conditions the applicant has for further appeals or administrative review if the situation applies, steering the applicant on what to do next.