I mean running and youre own vet clinic, store, or bakery, those are literally side jobs, because they still have to go to their real jobs. Where as Dream Home Decorator and Paranormal investigator is their full time job. Not sure if they intended that or what, but
I know when I hire an entertainer, they choose sims of mine in other households who are musicians and comedians.
What I think they are technically doing with this side hustle option, is for you to try to establish your own jobs at home, then if lucrative, you can quit you day job and work from home all day.
Farmers can do this from selling their crops, the Trendi, both of those make a ridiculous amounts of money if you put your mind to it. The other one I found was the artifact table, with the archaeology skill all the way up, can pay pretty good as well. I think you can also sell your baked good from home to a bakery already. You can sell paintings, photographs and stuff like that too but the day job maxed out is a lot more money and stable. Also royalties from writing, producing, mixing music from home. Royalties from the social media videos you upload, if you have that sector available to you in your fame path.
New one I found with famous sims, you can launch your own brand, I'm not sure all the categories, but my 3rd time starlight accolade award winning sim recently had this option presented to her on the social media table and she launched her own beauty cosmetic line. Note sure if it's an update, but I love it!