I'm looking for an app to browse YouTube using multiple accounts. It has to be convenient to switch between multiple accounts, and keep them separate. I've struggled to find a YT client that does this well, most of them seem to advertise "Use YouTube without an account". I'd look on the app store but frankly the play store sketches me the fuck out, even though google has a "moderation system" (we all know how well they keep malware off the platform)
Most open-source clients will by default have the option to block ads, which goes directly against the TOS and will be a quick way to get their API access shut down. This is why most open-source clients will avoid using the API in the first place, since they'll get shut down. This means that connecting to a YouTube account is not an option.
What you're searching doesn't exist and will never exist.
You want to comment/interact with multiple accounts, and that can be done exclusively with the official app and the official website.
The reason is that if someone reverse engineers and spoofs a way to comment from a foss app, 30 seconds after the first release, some spambot asshole would immediately edit the code to send spam, and Google would patch that way immediately
Not really anything like this out there, Grayjay might work but I've never tried it. You could have two different ones, like NewPiped and Libretube, maybe?
Unfortunately I haven't been impressed with the TRULY open source options. There are two KINDA open source options that I would personally say are much better:
ReVanced is sorta open source, but it's basically a tool to modify the YouTube app, which is not open source.
GrayJay is source available, not truly FOSS. But it's an excellent app and has been stable in my experience (especially impressive as it's only a few months old).
I second this, grayjay.app is great. It does break TOS but you don't have to login to the app, you can just add your subscriptions over and boom. It's not FOSS but it's not really too much of a problem since it's supported by an organisation focused on decentralizing the big tech (FUTO) so I think it's pretty safe to support using the app.
Personally, I really enjoy LibreTube (Website, F-Droid link, GitHub). It is great for privacy as it proxies all traffic to YouTube through a Piped instance meaning you don't have to connect to YouTube servers and your identity won't be exposed to Google.
Google killed off most YouTube apps some years ago, in favor of YouTube Red. Google stated those apps being in violation of the YouTube ToS, but even when those apps removed the violating features, Google would find new reasons for keeping them off the Play Store. Well, and then they would sell those supposedly ToS-violating features themselves, as part of YouTube Red, like for example background playback.
Since then, most YouTube apps are knowingly in violation of the ToS or at least not holding their breath that Google might decide so.
So, either they don't use the official YouTube API, like NewPipe and LibreTube, meaning you can't log in with those. Or they don't put in too much effort, like ReVanced just being a mod of the official app, so that won't have the features you want. Or I guess, they just risk it for quick profit, like Grayjay, but that will probably get shut down before they have such features developed.
Theoretically, a non-ToS-violating app can exist and could be distributed via the Play Store, but it would basically not be able to integrate any unique feature.
So, yeah, as others said, I don't think this exists.
You could try some alternative methods like:
Use two different apps, logged into two different YouTube accounts.
Utilize an Android work profile to be logged into different accounts with different installations of the same app.
If you just care about having different subscription feeds, use RSS feeds instead.
See if a non-Google operating system is less shit at this.
Well, reading that back, the above comment is maybe a bit harsh, because I hardly know anything about Grayjay specifically. They do seem to have a grander vision where they combine all kinds of services, not just YouTube, and maybe they really are hoping that Google won't ToS them.
But yeah, the way I imagine this will go down, is that Grayjay will grow for a bit, until Google notices losses from this new competitor. Then Grayjay will receive a letter that they're in violation of the YouTube ToS. Grayjay will try to get that resolved, but no one at Google responds. Eventually they'll be forced to take out the YouTube integration, making the app significantly less useful, which is its death sentence.
I don't quite understand how their monetization model works, so I don't know who will lose money here, but I imagine someone will.
Ultimately, they're building a business reliant on Google, which has never been a good idea.
I'm assuming because they aren't foss check their license) and clearly intent to have a business around the app. On the website it says they make money by selling licenses but I didn't do enough research to know how that works.