This may be difficult to do now that you already have thousands of records in both lists. The best way to relate lists together in SharePoint is via Lookup Columns. Think of Lookup columns kind of like a foreign key relationship in a database. But unlike a foreign key, its not something that you add to an existing column, it is a separate type of column.
If you were setting up two new lists, you would start by adding a Lookup column to your
Contenedores list that uses the Library MBL as its lookup reference, creating a many-to-one relationship from
Contenedores to the Library, and then making that Lookup be a "Required" field. Then users entering new items in the
Contenedores list would be forced to choose the MBL from the Lookup dropdown field to choose which Library document it is connected to.
In order to do this now that you already have thousands of items, you will need to add the Lookup column as described, but initially make it a "not required" column. Then you would need to write a script to iterate through all items in the
Contenedores list and do a search to find the file in the Library with that MBL, then use that file's itemId to set the Lookup column value for the current
Contenedores item. Once all items are updated, you can then make your Lookup column Required, then remove the "old" MBL column from that list.