Hi Aidel,
Basically, we will implement it using two OOTB methods: calculated field or workflow.
Here, I share the steps of the calculated field.
Background: Product entity has a field "Tax Code".

1) Create a calculated field for the Quote Line entity.

2) Click Edit to edit its logic. Set its ACTION as productid.new_taxcode. Save.

3) Add the calculated field "Tax Code 2" to the Quote Line form. Whenever we select Existing Product and save this record, the field "Tax Code 2" is automatically populated.
