You can compare two Salesforce Orgs (Production vs Sandbox or Sandbox vs Sandbox).
You need an Org Model for each Org you want to compare. When a PRO space has an add-on Salesforce connection license, you can connect one Production and up to 6 associated Sandbox Orgs.
A PRO space can have multiple add-ons and so you can have multiple Production Orgs and related Sandboxes. Then you can also compare Production vs Production.
Here is the support article on creating a Org Model for an Org
Open a Salesforce Org Model, run a report "Salesforce Org Model comparison", and select which Org Model to compare the current one to.
You will be notified when a new report is generated. Open the new report to see the difference between the two Org Models. The subtitle of the report will have a summary of differences:
how many nodes are missing in the second Org Model
how many new nodes were created in the second Org Model
how many nodes are different between the two Org Models
Click on the report to open it. The columns can be grouped, dragged and filtered. The report can be exported to XLS.