I Have What Appears To Be A Bot Registering As A User

I have what appears to be a bot creating fake users in my WordPress users. The role they are getting is Subscriber.

First, HOW are they doing this?

Second, how can I stop it?

Third, WHY would they be choosing the Subscriber Role?

I am relatively new to the deeper levels of WordPress, but I want to learn, so that I can give my customers the best experiences possible.