Even though it's tricky to set up for newbies, RetroArch is the ultimate solution for Android, since it has cores that aren't available standalone.
If you need to control your PC remotely from your phone, TeamViewer is an option.
XDA is the site to go if you want to get into Android stuff deeper. Get the XDA Labs app for the forums, news and apps not available on the Play Store.
Speaking of XDA Labs and its app store, get YouTube Vanced. It's a modded YouTube app with a full black theme, background playback and no ads. You don't even need to buy anything to use it.
BlackPlayer EX is my favorite music player. It's stable and easy to use.
Tachiyomi if you're into manga.
Profiler might be useful if you want to monitor CPU usage, temps, etc.
Blokada is the best ad blocker for non-rooted phones. If you're rooted, use AdAway.
Naptime and Servicely are the best for saving battery. They're more aggressive than Greenify and work much better, but Naptime bugged the status bar on my Nexus 5X. Didn't happen on my other phone, though.
SystemUI Tuner requires either root or ADB to set up, but you can make the status bar clock display seconds for example.
Smart Launcher has been my launcher of choice for years. Even my mom uses it.