So if you’re making something for someone and it sits in their house it’s probably fine. If it’s in a public space it really depends and it’s easy to protect yourself. When I did websites I had in our contract that we were executing the designs under the direction of the client and any licenses, rights, or claims were solely the responsibility of the client. Now, if you stole a logo design and the client didn’t know it could blow back on you.
If you’re by yourself stay a Sole Proprietor for a bit until it gets “real” then do a LLC. Investors prefer you to stay an LLC/S-Corp. until your series A (generally). The downside is as a partner you’re directly responsible for the income tax which gets super confusing at tax time. When/if you go C-Corp go Delaware C. Just save yourself the hassle otherwise your first big investors will make you do it anyway.
Copyrights in the US are inherit for created works. As soon as it exists it’s copy written to you but it has to be a creative work, not just a grocery list. Often it takes being published to firmly establish dates, and the “mail yourself a letter” thing doesn’t hold up in court. Although I have heard a github commit has held up… Here’s a link with a bit more on copyrights.