On iOS: Safari > Lemmy website > Hit share button > Add to home screen. On android, I assume steps will be similar.
I wholeheartedly recommend PWA app as such is published by the publisher of website itself, and I don’t need to download it either from official or 3rd party store.
In case you don’t know, PWA is basically a web app, running using your browser engine, which has almost same look-n-feel of native app.