How are fake followers built?

People sell thousands of (fake) followers in twitter for a few bucks.

How do they create fake account and fake followers?

Does it have any benefit to have fake followers?

They use bots to create accounts, i'm no expert in that field but i think something like Ubot can do those things.
To answer your second question about the benefits. It is known as seeding, that is when website owners launch a new site and add twitter and facebook acounts it's better to have 10,000 followers than nothing. It just looks better and people have a herd mentality, that is people follow people.
Imagine 2 bars next to each other.
One is packed and a que is outside waiting to get in
The other is empty
Both bars are as good as each other - Which one would you want to go into
Human nature would say the busy one, it's the same with twitter and facebook.
accounts with lots of followers look like the site is popular

in my eyes the biz has changed in ~2014

from
new created accounts -_- the most suspended because of this
to
hacked inactive accounts ~70% eggs, labeled as "mix"

the advantage for seller + buyer
the loss is ~10 to 15% -_- because of the given free EXTRA, the paid number "followers" will remain in the account

My question: is it so easy to hack twitter accounts?