[MishaalRahman]

You can just drag and drop APK files onto the scrcpy window to initiate an install (it just does 'adb install' basically).

[solstice]

Does this mean i can install signal on my tablet?

[Tajnymag]

As long as you can somehow receive the registration sms code on a different device, you should be able to. Mind it would deregister Signal from your phone though.

[4gotunameagain]

The real question is, where do you get trustworthy APKs, if you cant download them from playstore ?

You can already install an APK through the phone

[PufPufPuf]

Some devices don't have the UI for side-loading apks (TVs etc.). You can download apks from the publisher's website, 3rd party stores like F-Droid or IzzyOnDroid, or use Yalp Store to download directly from Google Play.

[IYasha]

Google Play doesn't really guarantee safety. It can promise though :)

[rand0mx1]

From aurora store, an alternative front-end for play store available on f-droid.