First things first, let's make sure all your databases are connected to Xano. Here's how you do it:

1. Head over to the "Connections" tab in your Xano dashboard.
2. Click on "Add Connection" and pick the type of database you want to connect (like MySQL or PostgreSQL).
3. Fill in the details like host, port, database name, username, and password.
4. Give the connection a test to make sure everything's working.
5. Save it.

Now, let's set up the data sources. These are the tables or views you'll be querying:

1. Go to the "Data Sources" tab.
2. Click on "Add Data Source."
3. Choose the connection (database) and then pick the specific table or view you want.
4. Do this for each table or view from all the databases you plan to join.

Next, we'll create API endpoints to fetch data from each table individually:

1. Navigate to the "APIs" tab and click on "Create API."
2. Define the endpoint URL and method (like GET).
3. Add a query to fetch data from the first table.
4. Save the API.
5. Repeat this for each table.

Time to combine data from different databases using custom functions:

1. Go to the "Functions" tab and click on "Add Function."
2. Give your function a name and head to the function editor.
3. Use multiple "API Call" blocks to call the APIs you created in Step 3.
4. Gather the results from these API calls into variables.
5. Use "Join" or other logical operators to combine the data as needed.
6. Write any custom logic or transformations required to process the combined data.
7. Save the function.

Now, let's create an API endpoint that uses the custom function to perform the cross-database join:

1. Navigate to the "APIs" tab and click on "Create API."
2. Define the endpoint URL and method (like GET).
3. In the API editor, add a "Function Call" block.
4. Select the custom function you created in Step 4.
5. Optionally, add any additional query parameters or logic needed for the API.
6. Test the API to make sure it returns the combined dataset correctly.
7. Save the API.

Finally, let's validate the entire setup by implementing and testing the cross-database join:

1. Use tools like Postman to make requests to the composite API you created in Step 5.
2. Review the returned data to confirm that the cross-database join is working as expected.
3. Debug and refine the logic if necessary.

And there you have it!