Venues on Reciprocal Reviews define roles: reviewer, program committee member, area chair, and so on. Volunteering means telling a venue which roles you are willing to take and how much you are willing to do.
Bidding for a role
On a venue's page, each open role shows what it involves and what it pays, if the venue uses tokens. For each one you can express a preference, typically something like preferred, if necessary, or no, and say how many submissions you are willing to handle.
Some roles are invite only. You will not see a bidding option for those; instead an editor invites you, and you accept or decline.
Being assigned
Editors do the assigning, not the platform. They use your stated preferences and capacity as input, along with expertise and conflicts of interest. You are emailed when you are assigned a submission, and again if you are removed from one.
Reciprocal Reviews does not host the reviews themselves. The actual reviewing happens in whatever system the venue already uses.
Getting compensated
If the venue uses tokens, you request compensation after doing the work by following the link in the venue's own emails back to Reciprocal Reviews. Compensation is pull-based: nobody watches for you finishing and pays you automatically, so if you don't ask, you don't get paid.
Declaring a conflict
If you are assigned something you should not review, declare a conflict of interest on the submission rather than quietly declining. That tells the editor why.