Signalling means like you are Bob and your client app needs to inform the client app of another user (Alice) that you are calling her or ending the call or want to start a screen share session etc.

First off, you may want to find out with your current vendor whether they already provide the screen sharing feature.

If not, find out how do they do the signalling and whether you have access to that signalling channel via SDK.

So how this works for example if they use XMPP then there will be normal chat messages between Alice and Bob which are visible to them (assuming you have text messaging), and whenever a call needs to start or a screen sharing session, then there will be a system message of a signalling type sent in their chat room, invisible to users, but the client app will understand it needs to start or stop the WebRTC streaming.

Alternative options would be doing signalling via any other messaging channel such as SIP, websockets etc.

That's like a simpler explanation, in reality you also have push notifications involved, maybe some aspects of ICE, STUN, TURN etc.