Suppose you have the domain mydomain.com, and you also have your blog running at mydomain.com/blog. What if you want to have your blog url be blog.mydomain.com instead of mydomain.com/blog? One way to solve this problem is using a synthetic record in Google Domains.
Here is my setup below for blog.gbt.codes
This makes blog.gbt.codes forward to gbt.codes/blog. The path forwarding option makes it so that a url such as blog.gbt.codes/myarticle forwards to gbt.codes/blog/myarticle.