Do anyone know how to start a Airbnb business

Depends on what you mean by an "Airbnb business". Are you talking a single room/house or multiples? Do you own them or have legal rights to rent a room/the whole unit/house?

I had one for several years and it isn't all that difficult to get it going, but there are rules, city ordinances (in many cities), etc.

I have 6. Best thing to do is go on Google. There are rules and safety things you need to do and get advice on accounting.

I presume you apply to the site after having a suitable rental space available..

Air up a jumping castle and charge people to stay in it

Just register on airbnb