If it’s a web site you just go to the url and log in …. No downloads required

I know you can “side load” apps but I am not familiar in the process.

Some here know how!

https://sideus.app/

At least you can add a website to your homescreen via safari

Sideloading (alternative stores to App Store), or using apps as progressive web apps (PWA) (websites that you can add to your app list)