There is a business rule named 'Assign from stock' which runs on requested item (sc_req_item) table that will populate 'Assigned' field on cmdb_ci and thus also for the corresponding asset due to asset-CI sync from the Asset-CI field mappings table. This is explained in the following URL:
https://support.servicenow.com/kb?id=kb_article_view&sysparm_article=KB0718514
There seems to be no other auto-population logic for 'Assigned' field. 'Assigned' field does not populate when the 'Assigned to' field is populated manually in alm_asset or alm_hardware.
