For about 10 years I worked exclusively temporary jobs, so I was the employee of temp agencies. Had no medical I was aware of. The benefits were nil, the pay barely enough to live on the cheap, partly because AFAIK there just wasn't enough demand for unskilled labor around here at the time, probably still isn't, don't know. The benefit was really getting a look at myriad working situations in my vicinity and meeting lots of different types people.
Later, after I'd mastered a programming language I got a few jobs by virtue of head hunters calling me, which was infinitely better than what I used to have to do, call a handful of temp agencies practically daily when I was unemployed and usually get told they didn't have anything for me.
I'm de facto retired now (don't need the bucks), but my last job was for an agency out of state that had me work mostly telecommute for a company around 50 miles away from me. The agency paid me and pretty well by the hour. Occasionally I had to drive to the location. Again, no medical or other benefits I was aware of. It was just part time. I don't believe I ever personally met any of the staff of the company that employed me on that job!
My first job after college was through Kelly Services. For tax purposes, I was a Kelly employee. I got my paychecks from them, all the normal taxes were withheld, and I received a W-2 from them.
I probably worked for Kelly. My most frequent temp agency gig was with Manpower. But there were others. I'd send in my forms with hours worked end of each week. Checks and W2s were always from the agencies.
My first programming job was for a tiny company that contracted workers for a large corporation. They had a little office in their facilities. After that job ended I started getting "regular" jobs where I was the employee of the company I was working for.