If I get certified from the online massage course on Udemy, does that permit me to legally charge by appointment for massages that I give at a private residence?

You can go to private homes to give massages. I researched laws because I did that for years before I got my training and lost a corpirate career. Cops can't bust you in a private residence. If I wasn't licensed and did massage out of my apartment, I don't own my apartment and there are regulations about doing commercial business on that private property I don't own so it can be risky if someone reports people coming in and out of your place.

You'll be legally fine going to someone's place. However, always be aware and have something like pepper spray on you for safety.

Also, those aren't real certifications on Udemy. Real certifications cost thousands of dollars and take a while to complete. You can still use that info and lie that you're a certified massage therapist to seem legit if you want. No one really asks about credentials.