Table to Table Relationships between two apps

There are two different approaches you can use here. You haven't really given an example of what specifically you're trying to do, but let's say for example you had a table of customers in one application. You can have a cross relationship where you enter say an order in one application and have a Cross relationship to select a customer and then you would have look up fields to bring down all those lookup fields automatically into your orders table.

That does create a dependency between those two apps and a few were in the situation where you have hundreds of thousands of records and a lot of users than you can run into performance issues down the road.

The other approach which maintains the apps being separate is you can have what's called the connected sync table. So for example, your source of truth for customers would be in one app and you create a Connected sync table in the other app and then every hour the data quietly mirrors across an updates on its own. So now both apps have a complete customer list but only one is the source of truth and can be updated.