Hello @YouTube! Am I just clueless or is there not a way to comment on videos you’ve watched via your phone? Every time I try and find it, I can’t and give up. It’s prob right in front of my face, isn’t it? @YTCreators #YouTube
Make sure you're logged with a Google/Youtube account. Load a video. Put your phone upright (portrait) and scroll down past the description. Keep going, past the suggested videos. See the comments? The very top line is where you can input your own comment.