I had my phone undergo factory reset because there was some kind of malware and worms but it didn't work so I had it professionally fixed. And when I was going to re-install an app but it's already removed from the play store and I don't want to download from unknown sources. Is there a way to retrieve this? If there's none, what could be a good alternative for Google Play? I'm scared from unknown sources because of what happened to my phone before it was fixed.
I think you just type the App name on play store and search. That has been removed or couldn't search, there is nothing you can do. Google play store's Apps are always update with new versions because the security reasons. But you don't worry. Apkpure is the one of large App store similar Play Store and you could search that app on it. It is fully trusted and they are always checked with Apps for worms, and malweare. Here is the link https://apkpure.com
The "Library" section of "My Apps and Games" lists all apps that you have downloaded in the past, even if they have been removed from the Play Store.