Welcome to the Airtable community!
There are two different issues here
- creating a login system for users to view their own data
- consolidating information from multiple bases
There are many 3rd party systems that can build a ‘portal’ for people to view their own data. Some common optons include MiniExtensions, Stacker, Softr, Pory, and Glide. They all work slightly differently, have slightly different features, and have different price points.
Showing data aross multiple bases is a different matter. While portal systems can show data across multiple bases, it is much easier if all the data is in a single base. However, you may run into other issues when trying to consolidate data, such as versioning information and dealing with record limits. (If a person changed addresses over the years, do you store each year’s address, or only the most recent address?)
There is no single answer or solution.