DSLRs perform better recording video in low light than phone cameras 99% of the time. This has to do with the sensor’s size and the ability to add interchangeable lenses to a DSLR. Another contributing factor is the ISO range, which is relatively limited to phone cameras.
But if you are a beginner and do not want to invest in DSLR cameras early in your YouTube career, you can always use your smartphone to shoot and edit your video for YouTube and slowly upgrade to better DSLR cameras.