I would like to work with a friend on the same bib file.
What online free platforms can we use to share the file and in what ways can jabref update a local bib file with an online updated bib file (by another user) and vice versa OR is the update process not possible in jabref, so it has to be done manually by a diff tool we both run on our computers.
There are various possibilities.
Here are the docs: Share - JabRef
If you are in a small setting and work with collegues you trust, sharing the file on a network drive is the easiest way to share a bib file. There are also programs that can help you sync the files between your local and remote storage location (e.g. syncthing, samba, rsync, …).
This means, in principal, you could use also use programs like google drive, one drive, drop-box etc. to store your files in the cloud, but those options hold the same limitations as your local shared folders (and some more, as depending on company policy, could in theory scan everything you do #privacy #copyright)
If you want to keep track of changes and instill more trust into the process, using a “version control system” like git or mercurial will enhance your workflow and is already a little bit more professional. If you do not want to set up your own local version control instance, there are dedicated websites (e.g. github, gitlab or gitea, … ) that offer easy integration with these tools, but if copyright and privacy is a concern, you may have to choose wisely.
Finally, we have those JabRef users that store their library file in a SQL database, which offers, at the very least, password protected access features. I have not yet used one of those, so it is hard for me to estimate its further benefits and functionality.