I'm not 100% sure how it works on all circumstances but this is the general idea about sponsorship. Basically, companies usually look for riders who compete in various locations and preferably win or place in their classes consistently. By doing this, their products are exposed to different audiences and are worn by riders who are talented and are able to produce results thus giving their brand a good name. Some companies may sponsor riders who have a large following on social media even if they don't compete a lot because their products are still presented to a number followers on the internet.
Some companies may approach you directly and ask for a partnership whilst others you can apply for a sponsorship. It really depends I think but of course being sponsored and having support helps tremendously! Good luck! :)