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.

There are different options here depending on your end motive. 
Do you want the data in Sales to read only or changes can only be made in the operations? 
Do you intend to keep both in sync?
- Use Pipelines with a "Record Added / Updated" trigger in Sales App → "Create Record" in Operations App.
- Use cross app relationship if you just want to view Sales data inside Operations.

Option 1 (Pipelines) automatically copies data by creating new records in the Operations App whenever a record is added or updated in Sales. This gives you full control, supports conditions, and allows Operations to have independent records with extra information. However, it creates duplicate data that needs maintenance.

Option 2 (Cross-App Relationship) creates a live link between the two apps without copying data. In Operations, you can pull real-time information from Sales using Lookup and Summary fields. This maintains a single source of truth and avoids duplication, but it doesn’t automatically create new records in Operations and has limited editing capabilities.

Just know the end goal and then chose one option.