For a 3PL that already holds your catalogue, Match Products (Products page > Perform Operation, or the button on the instance) pulls the 3PL's full item list and auto-matches it to Odoo products by SKU first, barcode second:
Example - onboarding. the 3PL already stocks 204 of your SKUs. Run Match Products: 195 match by SKU and become Synced without a single API write; the log lists 9 unmatched codes - 6 turn out to be retired products, 3 are typos in the 3PL's catalogue you ask them to fix. Then use Add in Bulk for genuinely new products going forward.