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Android is considered a open source project https://source.android.com/

In most cases people buy a device with Android set up by the device manufacturer. The device manufacturers however take an Android setup from Google and add their device specific SW.

The standard way to look for apps is the built in Google play store, however also a web browser on a PC with a big screen using the link https://play.google.com/store/apps can be used. Additionally any apk package file can be installed, if it is trusted.