where are your tables? Suppose they are on Google Sheets.
When you enter AppSheet and choose to start a new App, you are given the option of using your own data, from where you are taken to a window where you can select where the data is (GoogleSheets) and then select the spreadsheet that will be used as your database.
Once you select a sheet and the tab, AppSheet automatically uses the first line of the sheet as fields headers in a classic database.
you can add several tabs of the same sheet, each one will be used as a database.
or you can add more sheets and their tabs…
or with a pro plan you can add tables from tabs inside GoogleSheets, from Excel files in your dropbox, etc.
and use all of those tables as well as make connections between them.