How do I edit the drop down 'Sign In' form on One Page checkout for guest customers in Magento 2.2?
If you want to know the path of the file, you can enable template path hint from admin, it will display the file path in frontend.
To Enable template path hint Go to Admin panel, then Go to Store->Configuration. Now from left panel navigation select your store from top store view dropdown.
now under ADVANCED select Developer. After that Under Debug Section set Enabled Template Path Hints for Storefront to yes
The login pop up comes from below file in luma theme