I want to change the content of this page: /customer/account/login
Which file in magento2 is responsible for the this page?
You can enable template file, so you can know which file called
Store > Configuration > Advanced > Developer > Debug > Enabled Template Path Hints for Storefront > Yes
Hope it will work for you!!
If it work for you then mark as solution.
For customer/account/login action contents below file from Magento Customer module,
This action contains XML file inside below path,
Path of template file is,
You can find details of title like , New Customers, Create an account
Above template file is responsible for content of login.phtml file
Above template file is calling block function from below Block class,
If issue resolved, click Kudo and Accept as solutions.