in a hot water bath for canning, do you need a rack under the glass jars?

It's preferable, because you want the water circulating all around the jars to help with the sterilization and sealing process. You absolutely cannot put the jars directly on the bottom of your canning kettle because there will be a heat differential between the bottom of the jars and the hot water circulating around the rest of the jar; if you did, most (if not all) of your jars would break as a result. However, if you don't have a rack (they aren't expensive), you can fold a terry-cloth dish towel a couple of times and put it in the bottom of the pot, then put your jars on the towel. Perhaps you have a friend or neighbor you could borrow a canning rack from? It's also much easier to get the jars out of the water after they are sealed if you have a rack.

But if you're going to be doing canning with any frequency, I would encourage you to buy a rack. Also, I'd recommend you look into a steam canner. I don't know how old you are, but hauling a full canning kettle of water between the sink and stove gets really old, and when the water's hot, can be dangerous. A steam canner doesn't require much water at all, and steam is much hotter than boiling water, so does a better job of sealing the jars.

I use a silicone pot holder, the non-fabric kind that feel like very soft plastic

You do want something under the glass to lift it off the bottom of the canner because otherwise the heat level on the jars is uneven and may result in breakage, plus, as has been pointed out here, the heat doesn't circulate to evenly process the jam. A tea towel solves the first problem, but it doesn't promote even circulation, so a rack of some kind is preferable. You can put a bunch of canning bands, rim side down, over the bottom of the canning kettle if you don't have a rack, but if you're going to be doing canning regularly, do yourself a favor and get a rack (amazon sells them) so you can also have the benefit of putting filled jars into the canner easily and pulling them up when the processing time is done.

Try using foil scrunched up. Just make sure your jars are upright. It's a hassle, but it does work. Good luck.

It helps when you have something underneath, a rack or folded towel. But the best is to buy a "steam canner".

the jars need to sit on something.

Ideally you should have a rack underneath so that the jar gets "cooked" all around. You can use a make shift rack. I use a very small cooling rack inside a large pot