Is it possible to allow players to create their own Matches and then display them in a Match browser for others to chose from / join? Is it possible to do the same type of setup with with a lobby that multiple players can join?

For single Matches, your players can create Matches using MatchMakingRequests. When you create a Match in the Configurator > Matches page, what you are doing is creating a Match type, which behaves like a class, or blueprint. Your players then create instances of this Match type by specifying its Short Code in the matchShortCode field of the MatchMakingRequest.

For a Lobby, you would need each player to perform a MatchMakingRequest using a drop-in/drop-out Match set to Manual. Then, after the initial MatchMakingRequest, each player performs a FindPendingMatchesRequest. This will return a list of all the active Pending Matches. You can then use JoinPendingMatchRequest to join those.