Vendors can use the Import by CSV section of the store’s back end (Catalog > Import) to add their new products and update their existing products.
They can also export their existing products to CSV via the Export in CSV section (Catalog > Export).
The process of product import and export for vendors is exactly the same as for the store administrator.
The only thing that makes product import and export different for a store with vendors compared to a store without vendors is that information about the product owner is included into the CSV file with product data. This is done using the column vendor which provides the email address of the vendor who created the product.
The vendor column is present in all product export CSV files, no matter whether they are created by the store administrator or by a vendor. However, a vendor can export only their own products, which means the export CSV file for them contains only their own products and their own vendor email address.
Using the vendor column in the import CSV file is not mandatory: even if a vendor uploads a CSV file with vendor information omitted, the products being imported will still be identified as owned by this vendor.