No do is not the “none superuser” version of sudo.
As the comment above stated it’s part of loops. But since loop syntax are built in to bash they are actually seperate built-in commands. Because it’s an available builtin autocompletion offers it to you.
The do command here is just a special command that expects a certain set of parameters and a done at the end this is why you get an error trying to run do alone because it’s not adhering to the syntax properly.