Back to Web Development…
Subdomains work as well as sub folders within a website. The only reason not to use subdomains is that it may be easier to maintain a single website, than subdomains that act like separate websites. - Subdomains vs. Subfolders in SEO
Link manipulation and spam and how they could affect domain authority - https://moz.com/blog/domain-authority-and-spam-detection
50 reasons your website may deserve a penalty from google: https://neilpatel.com/blog/penalized-by-google/
To read: