How to use subdomains on Vercel just with a domain that you purchased on Google Domains
For this I thought it was impossible to use subdomains without paying more, but not once you configure your domain on a Vercel project (using their very tutorial) it's possible to use subdomains on your other projects (that are hosted on Vercel)
Buy your domain
Use this domain in your vercel project like mydomain.com
Go to your second project and configure the subdomain like secondproject.mydomain.com