Hi @smk ,
Consider your lookup column name as "lookup" of "List2" and "Title" field of "List1" is source to lookup field.
You could use the below query to get data of look up column:
/_api/web/lists/getByTitle('List2')/items?$select=Title,lookup/Title&$expand=lookup
If an Answer is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.