How can I update/add an entity reference field on content A with content B when content A field ID is equal to content B field ID?

Question

I have two content types, Content A and Content B, which both have a field_code field.

Content A has an entity reference type field (field_content) whose settings are reference type (Content) and vontent type (content_b).

I want to populate Content A's field_content with Content B's field_content when Content A and Content B's field_code have an exact match.

I can do this in the UI by searching for field_code which is part of Content B's title.

As I have more than 500 content items to do I would like to do this automatically, I have looked at doing is via a custom module, node_presave, migrate yaml files and a preprocess function in my custom theme.

Note: Content B has been generate by a migrate YAML file.