You only need to register the first-level domain name, which costs dozens of dollars a year. The second-level domain name is allocated by the holder of the first-level domain name on demand.
First-level domain name: www.qbao.com https://www.lu.com/
Second-level domain name: user.qbao.com passport.qbao.com enterprise.qbao.com
3rd level domain name....
After the first-level domain name is registered, only the holder of the first-level domain name has the right to assign the corresponding second-level domain name, preventing the second-level domain name from being maliciously registered by others
For example, the brush wants to pretend to be user.qbao.com to defraud the user's login password, but it cannot register on the dns server