The firmware is open source. There are many apps, some are open source, some are freeware.

Does it apply to the meshcore.io app as well? From what I know (not a meshcore user currently) there has been a kerfuffle recently when a developer decided to fork and vibe code the project using Claude, then silently trademark the name and move everything to his own website. The original and legit site should be meshcore.io while the .co.uk site was created by that developer and isn't considered official nor reliable by the community.

Only the app store app which does not matter for usage. There are tons of other clients. Core firmware is Open Source.