Questions about an apk file remains

Good day! Every power user.

I checked inside my phone after installing the app.
and I find that that apk file still remain inside my phone.
As well as another apk files downloaded from another site except Google play store also remain.
Of course the apk files downloaded from Google play store has been removed, I could not find that.
Is it correct, and I’d like to know why.

If you don’t get an app from the Play Store, you have to get the APK from somewhere else (mostly download it). The APK of course remains on the device (download folder) after installing / deinstalling the app. If you get an app from the Play Store, see here:

Thank you for a clear explanation.