I'm trying to understand why, as it is happening to me right now in a team I'm guest (external) in, I can't accès or create files, whereas in this very team's general channel I can. I presume the issue is related to the SPO library behind the private channel. Does anybody know where the problem comes from?
Probably because the Team Owner has removed guest permissions on the folder itself on the underlying SharePoint Site.
This is done as follows, on the underlying SharePoint site, selecting the folder in the site collection, access settings > advanced settings. Break the inheritance and remove the permissions of guests. It's probably a private folder for the owner of the Team or for internal users only - and can only be visibile by them. I would check with the Team owner.
Hmm, it's unlikely to be default - 99% of cases is that it's the owners decision and done manually. Could potentially be a teams template with a third party app - but I have never seen that implemented personally