Please use Magmi to import/update product with custom field in Magento 2. Magmi is import product data fast and direct on database. Magmi also create Select box options value automatic.
Please referee sample CSV file for product custom field
sku price custom_field1 PR-MB01 31 abx PR-MX02 32 pqr PR-UG04 94 ty PR-UG05 100
Make sure if you have php version <7.2 then user below git repository for magmi download https://github.com/macopedia/magmi-m2/tree/82ae373630c5291fc72ce948935916a811dc4f8b
default Magento has strict requirements for the file structure, including column names. so if you have custom ones, the third-party tool like store manager might be a solution for you. you can simply map custom columns to the database fields at one of the import steps