Hi!
1. Which lecture are you on? Can you specify?
2. The injections can be done in any page as long as the user can enter any data, this will also depend on how the site is coded and if it’s vulnerable to the attacks.
3. Which site are you scanning? Almost any good developer will prevent the access to phpmyadmin page.
4. Which sites? Remember you can’t run any of the attacks against sites you don’t have permission to.

Let me know.
Diego