Brexit made no difference to passport checks. When the UK was in the EU, it was in one common area with continental Europe for Customs (inspection of goods) but it was never in the Schengen area which is one common area for Immigration (inspection of people). Pre-Brexit, when you travelled between the UK and continental Europe, you went through Immigration checks but were exempt from Customs inspection; today there is both.
This difference makes travel within Europe interesting as there are countries that are EU but not Schengen (e.g. Ireland plus pre-Brexit, the UK) and countries that are Schengen but not EU (e.g. Norway and Switzerland). Plus the European countries are not as anal about Customs as the U.S. and some other countries are so Customs is usually more of a "walk right by" unless you have something to declare or they ask you to stop and intra-Europe flights will have a mix of people who are subject to Custom inspection and those who aren't.
From a trip a few years ago, we flew from the U.S. to Frankfurt and then on to Stockholm. Immigration was cleared at Frankfurt as we entered the Schengen area (no inspection of bags and no need to claim bags and then recheck them). At Stockholm, after claiming bags, we went through Customs. It was nothing more than a sign saying if you had something to declare, go into the Customs office, otherwise if you had nothing to declare or were exempt, just walk by.
A few days later, it was train to Oslo. Norway not being in the EU, there was officially Customs arriving at Oslo but I never even saw a Customs office if we had something to declare. And two days after that, the DFDS ferry to Copenhagen. I did see Norwegian Customs watching people board at Oslo but at Copenhagen, again I did not see a Customs office. For the most part, they're looking for professional smugglers and just aren't concerned about the average tourist.