Getting input from multiple databases, PDFGeneratorAPI

Hey guys, I recently started to make an Tax Invoice app here and I ran into some issues I could help with. Q1.
I three databases in adalo (Collections) I want to take data from,
1. The User DB → Name, Address, Phone number, GST Number, Company name
2. The Customer DB → Name, Address, Phone Number, GST number, Company Name
3. Transaction DB → Invoice Id, product name, quantity, total price and so on
The user data and the customer data are entered into the database using individual forms on their
separate screens whereas the transaction data is entered into the database using textboxes.
In a new screen I want to show all the combined data to the user (in non editable format) and
simultaneously update the combined data into a database called “Invoice records”.
How can I do that?
Q2.
I am using PDF generator API [https://pdfgeneratorapi.com/] to make my PDF Invoice, and from what I saw I was only able to add one the JSON data from one of my adalo databases (like just the user DB or just the Customer DB). Is there any way to add all three database’s JSON details at the same time? (This way I don’t have to create the Invoice records database)