You can google:

are royalties subject to se tax

are film royalties subject to se tax

For example, read this, and you can find similar resources via the web:

https://www.thetaxadviser.com/issues/2013/dec/kelley-dec2013.html

Yes, it is subject to SE when it was the created property of the taxpayer, this is their business, and it is created for the purpose of sales and royalty income. Sched C.

No, it is not a business if you created one thing that sold (but wasn't created to be sold, such as art) or is generating income, but that is incidental and not your business operation. Sched E. Or, you bought the rights (creatively, you can buy a "catalog") and are leasing out the use for the income.

Think of oil and gas. If you are part of the development and production; or, have an interest in the success from being the surveyor, for instance, and you accepted a stake in the production instead of being paid at the time of services. Or, won that interest from gambling.