I would like to build a browser-based client using WebRTC to join video conferencing meetings (could be any VC Provider like Zoom, Webex, MS Teams, etc.) using their SIP URIs. So, I wanted to understand does the browser-based client that I am making, also needs to register with some SIP server and need to have a URI, or we can join a SIP call without requiring SIP Registration?
You generally* only need to register your SIP URI if you want other parties to be able to call you. If you're calling from a browser then it's likely you won't be receiving calls since the SIP agent will only be available while the browser tab is open. The best starting point for browser based SIP is jssip.net.
*Some SIP Providers do require a registration in order to place an outgoing call, however, that's due to their security policies rather than being required by the SIP protocol. In my experience this type of policy is rare and used by maybe <10% of SIP Providers.
(could be any VC Provider like Zoom, Webex, MS Teams, etc.) using their SIP URI
Of those I've spent a fair bit of time looking at MS Teams and am fairly confident there's no SIP option to call Teams Users or Meetings. The only option I know for Teams is the Microsoft Graph Communications API which is a closed source REST services based API. It does use WebRTC under the hood but the signalling mechanisms, which is where SIP fits in, are proprietary.
I don't know about Zoom, WebEx etc. but I suspect they may be similar and have their own SDK's.