Where to allow it: Settings → Apps → your browser → Install unknown apps → Allow from this source.
Play Protect may warn because this build did not come from Google Play. Tap More details, then Install anyway — that is expected.
Xiaomi (MIUI/HyperOS): if the install is blocked or stalls, turn off Security's "Scan apps before installing", and allow your browser under Settings → Privacy protection → Special permissions → Install unknown apps.
Android only — there is no iOS build. On an iPhone, open the web chat and add it to your home screen.
Updating? Just install the newer APK over the old one — no need to uninstall. The app can also notify you when a new version is out.