Something tells me that the thing about Google not allowing custom Andriod operating systems to install apps is not quite true. I don't know about this specific topic yet, but I bet that if I look into it, I'll find out that there's nuance here that isn't been correctly portrayed by your comment.

You need to allow Play the play store and it's services and those will wall you in. Many times discussed here: many banking, gov, health apps around the world are banning anything not blessed by Google or Apple and installing on a non blessed system will not allow you to use them. My bank allows a modern and supported android or ios phone or a Windows laptop with a biometric card reader. Pretty much locked in and all banks are following.