A couple of people mentioned Apple Wallet. Hey, I love Apple Wallet. I use it whenever I can, like you said, for boarding passes on Alaska Airlines, etc. My vaccination info and credit cards are in there too (I use Apple Pay whenever I can, for both the convenience and added security).
But.......
Apple Wallet is exactly why I wrote this post.
Because on my last trip to Europe, Apple Wallet was not the answer here. Not by a long shot.
Reason #1: Support
Many of the electronic tickets we got in Europe had no way to add to Apple Wallet directly. Small regional companies may not be set up for Apple Wallet at all. You should be aware that the electronic tickets you get in Europe, for transportation, events, and more, could be sent to you in all kinds of forms, depending on the company. Some of ours were provided in an email as a link to a web page, and you were to go to that page and use the QR code or bar code on that web page. Other tickets were sent to us as PDF documents sized A4 for print, with the QR/bar code small in a corner, so if you want to show that on the train, you have to enlarge the corner of the A4 document on your little phone screen so that the code can easily be scanned by the conductor.
So I normalized all of those variations so that if there was no provision to add to Apple Wallet, I made sure I had taken a nicely enlarged screen shot of any QR/bar codes that was going to be scanned, ready in my Photos app.
Here's an exercise for you: If you sign up for Italo train tickets through this very website (ricksteves.raileurope.com), how do you get those tickets into Apple Wallet? Answer: You can't. Italo provides the ticket QR code as an image that pops up on the Rail Europe website. (Trenitalia uses a PDF.) I took a screen shot of it so that when the conductor walks in, they don't have to stand there waiting for me to load that web site, sign in, and drill down to my reservation where the QR code is. Also because the Internet can be unreliable on a moving train.
Reason 2: Time and hassle
For many services, the only way to get the ticket into Apple Wallet is through that service's mobile app. If you are only ever going to use a service once, like some airline you are only using to do one hop within Europe and you might never use them again, you still have to jump through their hoops. You have to go find the app, download and install it, create an account and a password, verify the account, and then after you finally reach your trip record in the app, you have access to the Add to Apple Wallet button.
I watched a (non-techie) travel partner spend 10-15 minutes going through that process, because they reeeeeally wanted to add it to Apple Wallet. What did I do? Spend one second taking a screen shot the QR code in their email so that it was in my Photos app.
Again, I love Apple Wallet. It is worth it, when a service supports it, and you will use it more than once to justify the setup process. But not enough places support it yet.
Tech background: You cannot just add anything to Apple Wallet. Apple Wallet is secure, verifiable storage of credentials so that people can't just make up fake tickets and stick them in there. You can't take random images or documents and add them yourself. You have to go through an Add to Apple Wallet button that is expressly provided on an app or website. So some custom coding is required on the website end to get it working. That is why you don't see it for everything.