No they are (were) an actual native Android apps that install automatically without needing to go through Google play. They have a much tighter sandbox and restricted access to system APIs as a safety measure.
I just tried a bunch of apps that I swear used to have it, it all of them just say "Open in the real app" now. I wonder if low usage here also means developers stopped using them.
Basically what would happen is you'd click a link to Instagram, and instead of opening a web page, it would open the Instagram "app" instantly. If you then tried to do anything more advanced in the app (advanced decided by the developer) it would prompt you to install the app.
@Skyline Lime had it (probably still has it). I used to rent a Lime scooter just by choosing "try app". I would then enter the scooter code manually, and I could freely ride how much I wanted. It worked all the time. I'm no longer using this feature, as I have a new phone with a bigger storage, where I installed the Uber app, so whenever I would try the same way it would prompt me to use the Uber app to unlock the scooter.
App Clips. I’ve used these several times to pay for meals at restaurants. Those use Local for Toast. QR code on bill. Scan it. Check the items. Add the tip. Pay with Apple Pay. I eventually installed the app, but I think it still uses the clip interface instead of opening the app. Did it for parking once too, different app. My local Sunoco also has App Clip tags at the pumps, but I don’t need those. Definitely a self-serve deal in my experience. I’d see more if I went out more often.