Well, I've been there with having to implement Google's login outside Google, and I can say that it is most definitely non-trivial.
To get the official Google token, you must go through a massive wad of obfuscated JavaScript in a browser VM. It uses a thing called "BotGuard" for which barely any information is available online to prevent automated logins, and apparently the code changes on every request to the login form. I know this from poking around at the page source and seeing a blurb advertising their credential security division slipped in at the top, like they just know that the only way you're getting that token is if it's exactly how Google and that security division wanted you to, and they felt the need to inform everyone attempting to reverse engineer the thing by looking at the source what they think of their effort.
This is the reason why we don't have Google login in another unofficial YouTube frontend called invidious[1] and youtube-dl[2].
That's right, not even youtube-dl can handle logging in to YouTube since about April 2019. That was about the time that Google apparently got around to patching their auth endpoint so you have to run their obfuscated JavaScript if you want in. Since then scores of issues reporting 400 responses from trying to download Watch Later or private playlists were posted on youtube-dl's tracker, collected in the issue I mentioned. Their response? Close it as a duplicate without, well, any other response. None as of present writing, eight months later. Exactly the same response as nearly every other issue reporting the same error.
That is a misstep on youtube-dl's part, and has greatly affected my impression of the project's leadership, but part of me also thinks that they don't want to admit that it's nearly impossible to get past a system Google specifically designed to be impossible for third-party clients to get past.
Just a thought: maybe it could add a function to just sync user's subscriptions via scraping their user page? Of course this requires you to make it public, but at least it would provide an additional option.
I use YouTube frequently, but I almost always just watch my subscriptions.
By not having a login system, it is basically rendered useless despite how nice it is to be ad-free and having a much cleaner UI
(I knew you can import subscription, but I sub and unsub channels constantly so it becomes a pain very quickly.)
I'm currently using YouTube Vanced instead.