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

Does this mean i can install signal on my tablet?

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.

hmm.. my tablet is unlimited data 5g... I get txts on it all the time as its SIM inherited some random number that the previous person was on quite a few group txts.

Although, I confirmed that I can install anything or any OS updates I would like and the service "should work"

Its a great little device that costs me $15/month for unlimited data - so I downgraded my phone plan to the $30/month plan (I pay my cell plans a year in advance so I never have to worry about a monthly phone bill...) but now for $45/month I have both my phone and this tablet which is cheaper than the $50/month they wanted for 10 gigs of data on my phone plan...

now I just need to make the most of this Alcatel Tablet. (the battery life on this tablet is quite good!) but now I need to hot-spot with it over USB, but have to look at how to do so.

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

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.

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

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

it's called 'sideloading'