Grayjay is great IMO. It combines video from a variety of sources, such as YouTube, Odyssee, Rumble, PeerTube, Twitch, etc. I disable most of those, but I do have some channels on multiple platforms. Also, there's a feature to recognize the same content across services for the same creator, so users can choose the service they want the content from.
It's Android only, so hopefully they add more platforms eventually. One annoying thing is that it's not really open source, it's just "source available," but that's way better than the propriety platforms they're providing an alternative to. That said, there are a few things I don't like about it:
rotate doesn't work reliably for me - I just got a new phone, so I'll see if it's still an issue
seems like it uses way more resources than necessary (i.e. compared to NewPipe)
kinda buggy sometimes - it is getting regular updates, so I think it's just growing pains
And some things I love about it:
available outside of the Play Store - I get updates within the app, just like on a desktop app, or I can install through F-Droid (they have their own repo)
PiP - I like having it open while wasting time on Lemmy
no ads or other nonsense - you can buy a license, but features aren't gated behind paying (I'm planning to pay once rotate is fixed)