@MisuseCase @Mer__edith First step: In Android permissions you can with one "click" enable installing apps from one existing app. Got to settings->apps->Files (or whatever you have)->advanced/Install Unknown Apps and set to "allowed" and you can then install downloaded apps from the file manager.

In older versions, it was under "security" and would allow installing non-Google Play apps from any program on the device that could open them. This of course had the potential disadvantage of allowing silent installation of malware from the browser.