Do I need a VPN when entering passwords via apps, on a public wifi?

You create a false network that collects any information it can… even if they access a site with HTTPS, some packets might not get encrypted properly and it also could be possible to redirect and connect the user to the site without HTTPS

what uses TLS? the VPN?

some packets might not get encrypted properly

Sounds bogus.

it also could be possible to redirect and connect the user to the site without HTTPS

Yes, a downgrade attack. I think there are mechanisms to prevent this, and the browser would show it if you got HTTP.

No, I think MITM is not “easy to pull off”.

This is where Reddit sucks, people air their opinions no matter how ignorant

Then people who actually know what they’re talking about, like me who is skilled in computer science, information technology, cyber security, artificial intelligence among other things tend to get shouted down