It’s not available for new accounts created recently, only old accounts have access to Hobby. If you don’t have access to Hobby then that account can never access it as its too new.
Hobby requires a $5 commitment and includes free allowances of certain services as well as the $5 included usage. A list is here: Fly.io Resource Pricing · Fly Docs
Legacy Hobby has the same free allowances but no $5 commitment or $5 included usage.
Pay as you go doesn’t have free allowance or included usage. You pay for everything you use.
Some information is here: