>I’m not completely sure what distinguishes a website that has an “Install” option from one that doesn’t,[...]<p>You're talking about PWAs, which require things like a manifest and service worker to make them "installable" by the end user. See e.g. here:<p><a href="https://web.dev/explore/progressive-web-apps" rel="nofollow">https://web.dev/explore/progressive-web-apps</a><p><a href="https://en.wikipedia.org/wiki/Progressive_web_app" rel="nofollow">https://en.wikipedia.org/wiki/Progressive_web_app</a>