Hi @sebastian
Currently descriptions cannot be retrieved automatically for BigQuery:
https://github.com/metabase/metabase/issues/18872 - upvote by clicking 
on the first post
Alternatively you could either use the API (https://www.metabase.com/docs/latest/api-documentation PUT /api/table/:id and PUT /api/field/:id) to update the description or update the application database directly (look in the tables metabase_table and metabase_field).
Or use the hidden metadata feature, have a look in Sync semantic type from Postgres?