Varies per country and per system or even per line.
There are just a very, very few countries that have access-gates. Near universal-at least in all bigger stops, are the UK and NL. IN UK you always have to use manual check. In NL the QR code on the cover of paper pass-and likely also on mobile ones, works a very few times/days, but this has nothing to do with any REServ. If you find it does not work in NL-use the urgency help fone.
In ES there are airline style checks for AVE trains-when they will also check the actual booking.
In SOME -mostly major and start-stops in FR and IT for superfast trains there are gates -or more often manual checks with fast readers-they want to see the actual REServation, at least in FR these also have codes. And they will want to check the then current covid rules-prepare yourself for that (in IT its called green pass). Most often then there is no more on-board checks. Also holds for €*
Local cercanias trains around big cities in ES also have gates-no pass will be able to open these-go to counter and ask/show pass. They control remote. Same for around Lisboa/PT. NO staff? Bad luck!