TIPWithin automations, you can link records using the primary field value for any of the primary field types—expect the formula field type.
You can use both the "Create record" and/or the "Update record" actions to link new or existing records, depending on the workflow you are building out. You can link records together by using either 1. the primary field value of the record in the corresponding table you'd like to link to or; 2. by using the record ID of the record in the corresponding table you'd like to link to.
If you are linking a single record you can choose to use either the record ID, or the primary field value for the record you'd like to link to:
You can also link multiple records to a single record by looking under the header for "Make a new list of...", and choosing either "Airtable record ID" or "Field values":