for android apps, the mobilism forum is great, thoughb only questionably safe, I guess. it's all one click hosters, which sucks, but the apks are usually small enough in filesize to not make it a big deal. Just come armed with a good ad blocker, I guess. You can also try your hand at cracking/patching apps yourself, right on your phone with stuff like lucky patcher

for switch, I'm not sure, they'll usually be among other gaming torrents on all sorts of trackers in my experience

Are there any phone games worth playing? The whole market seems to be saturated with shovelware I've given up long ago.

I've always used rexdl, never had a problem.

Switch: NXBrew

Android: dFast, Happymod and if you can't find a modded version of it, try patching with Lucky Patcher.

You can also try platinmods, it's a forum, so most likely you have to register to see the links.

You can download any app hosted on the Google Play store from here: https://apkcombo.com/

Use Blokada 5 (requires sideloading) to block ads: https://go.blokada.org/apk5

Switch: use nxbrew. They've got basically everything.

For apps you could try Aptoide ig. Idk the state of the app currently but you can look up what people are saying about it in terms of safety etc