Sorry, I completely misread your question, and answered incorrectly.
For each user, what I do is make a csv string in firebase and call it all users or something like that. Then to display all the users data or to search in a list then use that. Now to find the data you must make sure that your objects and values follow a strict pattern/format, so you can simply substitute variables to find what you need.
From looking at your photo you shared, you will have a lot more loc123456… values. If a user wants to select specific possible locations, you must create another tag and make a csv string in this format:
This way a user can select from this list, and then following a strict format find what they need inside the users tag.
Is this what you meant?