How do freelancers structure their businesses/do taxes?

• As mentioned, do read the regulations on incorporation with the intention of tax breaks. But IMO nothing wrong with incorporating and having a more legitimate entity to do business as an agency rather than a single freelancer.
• Running a dropshipping/e-commerce business: definitely good to set up a company, probably a Pte Ltd in case clients try to make claims against products sold by you.
• IRAS will send you the reminder to file your taxes as a freelancer at the end of the Year of Assessment. Don't attempt to cheat on that!

p.s. I'm interested in dropshipping/ecom too and the challenges faced by such entrepreneurs! I'm also freelancing + doing biz. If you don't mind, we can connect further to exchange ideas & share our challenges. No pressure though. (admins remove this paragraph if not appropriate).

Iras has forbidden incorporation to reduce taxes if there is no other legitimate reason.
So incorporate at your own risk.