These are super cool parts of signal's architecture, that are not obvious to understand, but you can truly verify client side that (1) signal only sees an IP address, no phone number, associated with each outgoing message, and (2) signal has no idea who is in which group chat and which permissions you have in those chats.
The first one is pretty simple: you don't prove to signal who you are, signal just routes packets and lets the receiver verify that the sender is who they say they are by verifying a short lived certificate attesting your identity.
The second one is more interesting: group chats are implemented as a complete graph of direct messages between all participants. In order to update the group state, you send Signal a zero-knowledge proof that you are a member of the group, which convinces Signal that you can add or remove people, without ever revealing your identity. This same mechanism is used to prevent griefing, spam, and DDOS attacks for sealed sender.
Again, both of these can be verified by only looking at the client source code, and nothing else.
More info: https://signal.org/blog/sealed-sender/ https://signal.org/blog/signal-private-group-system/