How to generate an .apk file in Android Studio?

It depends on which apk you are referring to. Every time you compile your project in Android Studio and run it you are generating an apk. The difference is in the build configuration.

When you only compile for debugging, you generate a development apk, which usually stays in:

SEU_PROJETO\app\build\outputs\apk

With a name of type:

app-debug.apk

This apk should only be used for testing and development. You do not deliver this version to the user.

For final release version, you must make a digital signature on the apk, then send it to the store. To do this, go to Build > Generate Signed APK , according to the image:

So,youhavetosigntheapkbyfillingintheinformationinthewindowthatwillappear:

This key (password and file) must be well saved , as you may need it to make updates in the apk. If you lose this key, Google will not authorize the upgrade and you have nothing to do (other than registering another app).

Finally, after this window Android Studio will compile the signed apk and will open already in the folder where it is. This you can send to the store.