Deactivating a user will cause all of their Lead and Customer records to have their Owner set to Unassigned.

Some other record types will retain the user's name in the Owner field until reassigned.

To simplify the transfer of records to a new user, we advise creating a Work stack for each record type (e.g., customers, tasks) and filtering on the Owner field in order to show records currently assigned to that user.

You may then reassign all of these records before deactivating the user.

Follow this article on how to reassign opportunities from an Opportunity Work stack. You will have a similar option (either Reassign, Reassign Owner or Update Owner) to transfer records for Leads, Customers, Tasks, Quotes, and Sales Order stacks.