Some websites can be directly “installed” in you app drawer. They are really like an advanced shortcut for the mobile webpage. You probably haven’t seen it a lot because commercial webs that want to shove their propietary app down your throat disable this option.
But both Lemmy and Mastodon have it enabled.
To do so, click on options on your desired browser and you’ll find an “addto startup page” option or something along those lines.