“Is it possible to make apps available outside apple or the g00g store?”
I think you have to jailbreak Apple.
Android has an option to allow you to load apps not from the store, but it isn’t exactly convenient (overall, they don’t want you doing that).
For most websites out there, you don’t really need an app per say ... if the site is coded properly and knows that a mobile phone is communicating with the site, the site could enter into a “phone friendly” mode that would look identical to an app. They don’t do that for some reason though :-).
Apple threatening the Parler app may be a ploy to inoculate themselves against the RAT antitrust claims, which are based upon Apple’s anti-competitive*, bottle-neck*, market power* against competing apps.
*antitrust jargon.