To me it depends on which country you are traveling to.
For myself, the app for my airline of choice is key.
Since I've been to France most recently I also frequently used the G7 Taxi app in Paris and the app from the French train company.
If you are planning to visit museums, they may or may not have a good app. In 2019 when I was in the Netherlands the apps for the Rijksmuseum and Mauritshuis were wonderful. In contrast, I find the app for the Louvre confusing and useless. In 2018 Westminster Abbey had an app where you had their entire audio tour on your phone. I think the one for Stonehenge was good as well but that is a bit fuzzy. Anyway, check the big sites you will visit and see if there are apps sponsored by the museum or venue (not a paid app!).
Do you just have one phone for 2 of you? That can be difficult. For instance, for France you needed to access the health Pass Sanitaire on your phone for EACH person which were separate. You could print out the code or screen shot them into your photos so you could just go there for the restaurants to scan your QR codes if you just had one phone for 2 of you.
To me the critical skills are:
Being able to access an email for every person in your party - everyone needs a separate email account. For France each person needed their own email address to be able to get a health code.
Being able to take a screen shot. This means taking a photo of whatever is on your phone screen. This may be a ticket you have just purchased for a museum or a health code or even your boarding pass for your flight. You need to know how to access your pictures so you can find this screen shot. It is helpful if you learn how to add a screen shot to a "Notes" section (sorry, that is what it is called on an Apple product, not sure what it is called on Android). IF you screenshot something you do not need to access your WIFI to retrieve the information as you would if you are looking for something in email or on an app/website.
Practice surfing the internet from your phone so you can figure out how to buy museum or other venue tickets online.
Many use googlemaps online. I prefer a paper map but you can also do a route on googlemaps and screenshot the route so that you are not using battery when you navigate somewhere. If you intend to use googlemaps, practice with this ahead of time so you will understand how long your battery will last.
Good luck! Use your phone a lot at home so you are comfortable with it.