Do other nodes exchange data?

Assume I have a node A running syncthing and sharing a repository R in master mode. On another node B on the same LAN I connect to R. On node A I share the repo to B. The contents are now synched from A to B.

Now I add a third node C and connect it to R. On node A I share the repo to C. Are the contents are now synched just from A to C, or also from B to C (assuming B still has the unmodified contents from A)? In other words: do all nodes that carry a specific repository participate in data exchange even though they (officially) do no know each other?

No. Each node only talks to those nodes it has been configured to know about. It’s up to you to make that either full mesh (all nodes know about the others), a star (all nodes connect only a “central” node) or something else.

Thank you for the clear answer.