I agree with Jean on the need for a Smartphone. I ran into the need to scan a QR code for a menu in Bordeaux in 2021, so right after Covid. They were not handing out paper menus so you had to scan the code. I didn't have data and was not up to date on my phone skills so I just picked something off the chalkboard.
I've also run into the need to download an app or scan a QR code in Museums. In the Louvre's David exhibition last Fall you scanned a code when you entered the exhibition and got the audio guide.
As far as train tickets, as mentioned you can go to the train station and buy a ticket but might face a fairly long line or you can buy from a machine in the station. For a longer distance train you'd want to buy before you leave home for better pricing.
I stopped reading your link when she encouraged buying a Paris Museum Pass which is not really a good strategy any more except in isolated instances. It's much easier to buy a timed entry from the specific museums you want to visit.