View attachment 22462
Under the video, under the channel subscribe.
If it's not there it's either because it's on YT Kids or the uploader has comments off
If you're on browser, you have to scroll down past recommendations, or in the M (m.youtube.com) version, you have to scroll down on the left side, past the channel and subscribe buttons.
I don't use the app so if you're on the app I can't help you.