I’ve got some movies stored on the SD and also some on a thumb drive (USB A instead of USB C so can’t plug it into my iPad)
Can I basically use network share/SMB on PiFi with powerbank to stream those movies on a flight, even if there’s no internet?
Yes, you totally can!
You can use it in Files app on iPad OS (it generally works better for read (i.e watching) than write (uploading) on iOS/iPad believe this is an Apple quirk), can also use apps like Infuse for a richer movie experience.
No need for internet connection to access local shares at all so you should be all set!