Earlier quoted context omitted.
I'm not aware of any scare walls on iOS. On Android is a site is installable a little banner is supposed to pop up and ask if you want to install it. On iOS one has to open the site's share sheet and pick an install option nestled in there somewhere. I suppose that counts as a hoop, but how else would you surface the functionality without it being invasive and annoying for most users? I think a more pertinent problem…
Those are downloads though the app stores. The links merely funnel you though the stores. Native web downloads of apps are impossible on iPhone/iOS. You have to go through the app store. Native downloads of apks are possible on Android, but not until you navigate to the hidden settings and enable them. And even then, Google scares users from installing apps this way. Nobody in practice does this. It's effectively not…
This used to be the case for older Android versions, but isn't anymore. Currently any attempt to install an APK from an app without permission to do so will pop up a missing permission warning with a deep link into the relevant settings page with a switch you can flip. Flipping the switch will initiate the installation of the APK. I can't imagine sideloading being any easier without removing the permission barrier entirely, which seems ill-advised.