I liberally use the "Not interested" and "Don't recommend channel" options in the menu. Works okay for me.

These options are literally the only way for me to even get new recommendations. YouTube "helpfully" fills my recommendations with videos I've already watched, videos from channels I'm already subscribed to and watched, its own "mix" playlists of songs I listen to, and totally random "topics" it thinks I'm interested in. I have to "Don't recommend" every channel I subscribe to and "Not interested" every video I click on but don't watch fully, or they completely fill several pages of recommendations.

I don't know what happened or when, but it's impossible to discover new content there anymore.

There's also the option of going into your Youtube history and deleting things from it. More than once I've clicked on something out of curiosity and regretted it, only to have youtube recommend more of the same, but removing the original videos that caused it from my history seemed to do the trick. My theory is that some video categories have a highly addictive nature to certain groups of people. Youtube's algorithm picks up on this, and notes that people who watch x are likely to want to watch y and z etc. It could be conspiracy videos, pickup artist things, young women, or something feeding off of an insecurity. The algorithm doesn't care, but it can pick up on these patterns of viewership and entices you with more.

This started a couple of years ago. I remember quite distinctly when suddenly the suggestions were injected with things I already watched instead of actual on-topic suggestions. Tried disabling the watch history with mild success.

This was also around the time I just turned off the entire thing via DF YouTube :)

I now view YT like a dumb video hosting service; can't be bothered to fight with its stupid UI or inexplicable ML background fuckery.

Maybe some day they'll equip users with the tools that are appropriate to browse the biggest video platform of all time. They could take a hint from Steam on how to do that (or just use common sense)

Where is don't recommend channel? I saw a video about this when searching how to block specific channels but it doesn't appear in the UI of my YT android app...

In the browser it's in the "..." menu to the right of the recommended video's title. I don't know about the app.

I keep using them on this one set of day trading ads and they keep showing the exact same ad to me.

Don't resist it, they are right. You need it.