Bob, that may be part of the problem. To use iCloud, or syncing photos, or iCloud Drive you have to have an Apple ID and it has to be tied to a telephone. But as you add devices to sync, they, too, are added to the distribution list for the 2FA codes. So, for example, if a family has one AppleID to share, then when someone is out on the road with an iPhone and wants to access the account, then everybody else at home who has a trusted device on that AppleID sees the code come in, which might be confusing/annoying.
Frankly, I think everyone should have their own AppleID and then use Family Sharing to share what the family needs to share. That way if Dad needs the 2FA code, nobody else sees the code arriving because it only goes to HIS AppleID trusted devices.
Apple doesn't make it easy to ferret it out (Their legendary avoidance of documentation is way over done, IMHO) but with some digging at Apple.com for support documentation you can dig up the stuff you might need. Or you can get one of the "Missing Manual" type books where it's put in one place for you.
For example, here is Apple's documentation on 2FA:
Two-factor authentication for Apple ID - Apple Support
In the middle of the article is a great recommendation:
You should also consider verifying an additional phone number you can access, such as a home phone, or a number used by a family member or close friend. You can use this number if you temporarily can't access your primary number or your own devices.
Basically, they are trying to say that in addition to your one and only iPhone, you should consider a second device that could be used just in case the iPhone is dead, broken, stolen, lost, generally not available and you need to get to the AppleID. Now consider what that would mean: The 2FA codes would go to YOUR phone and also to that OTHER phone every time it was needed. That might be annoying to the owner of that other device. It is good advice to have a backup, but a better way might be to have a different device, such as your Mac, with Messages running and logged into the same iCloud account. That way the owner of the other phone doesn't get nagged with 2FA codes that don't apply. But one other factor is that if you now travel, but leave the Mac at home, then if you lose/break/get stolen your iPhone then the 2FA will go to your Mac at home, which is kind of useless at that point.
Bottom line, each person should consider HOW to implement 2FA for them and to pick options that make sense for them. Apple gives you options, albeit they are well hidden.