Drupal multisite on a shared hosting
There is no good answer for this, as it will vary host-to-host.

Sometimes, shared hosts use management tools that map domains to accounts to directories.  This makes it hard(er) to do the Apache ServerAlias settings.  I have worked with shared hosts that have let this happen.

Sometimes, hosts will map domains to user accounts and run Apache in FastCGI mode, which may complicate things.

You need to contact your host to see what is possible.