You don’t have to use the Play Store. You can either compile Signal yourself or use a trustworthy 3rd party build of Signal. Personally, I use Molly. It’s Signal for Android but with some neat tweaks. It’s not even available on the Play Store, it’s exclusive to F-Droid and Accrescent. You can’t do any of this with proprietary garbage like WhatsApp. Neither can you modify it to add features, nor can you look at the source code or compile it yourself.