It’s the swipes and the scrolling that I miss the most from a mobile app!
Reading a post, swiping back to /c/, reading another post.
One cool feature from Apollo was their GIF scrubber, you could move back and forth, play it faster etc.
It’s the same reason why I’d rather use Paperback on iOS instead of Mangadex on my mobile browser to read manga. Swiping on a phone is faster than pressing next or back.
For your question about how you turn a website into an app, the same way those 3rd party apps did it on Reddit. You can fetch the posts via Lemmy’s API and show them in a different way in an app :p (I hope I understood the concept correctly.)
I personally don’t use that website, but I hope you get well soon!