It just depends on your platform's specific TOS. I can have my work on different platforms so they all have to be paid. You can buy my books on Amazon in print or read digital copies.
The biggest piece of advice that I'm sharing these days is to think about your creations as a business. While it's totally fine and valid to post your stuff online for fun, if you plan on making an income online - you have to be thinking ten steps ahead. While I didn't use them last year, I'm totally planning on writing off my Canva premium bills and FB ad receipts as business expenses.
You also need to shop around for platforms. Tapas lured me in with the promise of earning money. While I stay for the community, I know that I'm not writing to a profitable niche here. But I'm on other sites that pay decent royalties and pay on a schedule so I can make my plans. Other sites hold tighter reigns, so it literally pays to do your homework.