This subject usually refers to one of two things: either accessing legacy YouTube features on an old Android 6.0 (Marshmallow) device, or (more commonly) it refers to specific "modded" versions of YouTube (like YouTube Vanced or OG YouTube) that were often distributed via bit.ly links and required Android 6.0+ to run.
If you are holding an old device running Android 6.0 and the official YouTube app from the Play Store says "Your device isn't compatible," you have two solid options. bit.ly youtube android 6
You can sideload the official YouTube app, but you must find a version compatible with Android 6.0. Newer versions may require Android 8.0+. This subject usually refers to one of two