By default, there are no rule, so at first the dynamic filtering pane can be used to see all the domains to which a web page connects.
From there, you may look where web sites connect, and work to block specific domains when you find out they are not really needed to make web pages render properly. I give an example here using Facebook domains which most of the time is really not needed for web pages to render properly.
As time go, you will have build yourself a list of rules to block undesirable domains which do not add real value to web pages – rather the opposite, they slow down page load, increase bandwidth consumption, increase privacy exposure, etc.
I personally do use medium mode, and yet I still follow my own advice here, because sometimes I may want to disable medium mode for a specific site, and thus I still benefit from my own blocking rules for domains I consider superfluous (examples of domains I block everywhere by default: