What are user properties?
User properties are attributes that can be used to describe segments of your user base, such as geographic location etc, pretty much like dimensions, but user-scoped. There are currently a number of default user properties such as age, app version/store, country, device brand/category/model, first open time, gender, interests, language, new/established and OS version. You can also add up to 25 uniquely named user properties per App+Web property.
Here is the full list of default user properties: https://support.google.com/analytics/answer/9268042
One thing you must understand about user properties, is that you need to preset them, and register or create them in the interface before you can use them.
There is also a limit on the number of user properties of 25 registered user properties per App+Web property, and a limit of 100 registered user properties per App+ Web 360 property.