"What is your preferred way to sit on the ground (with no chair)?"
Depends on the weather. If it's not raining, after my internal "take a break" alarm goes off, I'll begin to scout around for a nice spot to sit down. By nice I mean moss, grass, a log, a tree stump, or a flat rock. My favorite is probably a big mossy log or rock where the moss from the ground has grown like a big rug over everything. I'll check for bugs and such, then I'll just plop down right on the ground. If it's sunny and dry out, I won't get wet at all. If it's cooler and maybe rained the day before, it might be a little damp, but I usually don't care. I wear either thicker nylon hiking pants or thinner nylon running pants, so at worst the seat of my pants will be slighly damp, and this dries off after like 5-10min of hiking after my break.
If it is my ideal spot of mossy ground/log, then I use the log as back support. Otherwise I am fine without back support sitting cross-legged or with my legs propped up in front of me. If I need to rest my body more, I'll lay on my side with one elbow as support and alternate between elbows--but it's rare that I am this tired. If I am really tired, which again is rare, then I will break out my ground cover(s)--cut up garbage bag (40g) and/or small foam pad (57g or 40g depending on what backpack I am using)--and just lay down, maybe take a nap, and use my pack as a pillow.
If it is raining or if I am in a location that is less than ideal (i.e. no moss or grass), then I will look for a big tree to sit under. I big pine tree is usually the choice, and will have a bed of old pine needles under that helps with padding. I have found trees big enough and with enough branches to eat lunch under during moderate rain and be fairly dry. On some rare occasions there are other natural shelters I have also used to sit under, like under cliff overhangs or under lop-sided boulders. Depending on how soaked the ground is or how wet I already am, then I may or may not take out my ground cover--it's about 50/50.
If I am in an area, like a big hill or very rocky terrain, that lacks these more ideal rest stops to sit on the ground, then I will just sit on bare stone. If the stone is rough or of my rear end gets cold in cooler seasons then I will take out my foam pad--this is also about 50/50. If it is raining and I am stuck in this less than ideal terrain for a break, I'll just keep hiking until I'm back in the woods again--which is usually not that long, given my location.
Then of course there is just plain good luck. Sometimes on marked trails here there are trail shelters or benches, and I will pass them around the same time as I want to take a break.
In the winter when there is snow on the ground, well this makes things all the more easy. I just look for a spot that is flat, poke around for anything that might not be good to sit on under the snow, get out my small foam pad and sit down. I suppose I might as well explain my two different foam pads while I am at it, since I've already given a complete breakdown of my sitting on the ground routines and preferences. So my go-to packs are a Zpacks Zero (frameless, obviously) and a Zpacks Arc Blast. My foam pad for the Zero is a smaller foam rectangle that acts as my pack's support and so I don't feel my gear poking me in the back, and also acts as a sit/kneel pad as well as supplimentation to my sleep system (e.g. under my legs or as a stand-alone torso pad). My foam pad for my Arc Blast is slightly larger so that I can roll it up a bit and strap it to the base straps. Same applies to how it functions, only that with this pack I don't need any back support, of course.
This concludes the most detailed account of how I sit down out in nature I have ever written (or thought of at once), because what else am I going to do with a toddler that is taking their afternoon nap right next to me? Moving might wake them up, and I've already checked my email and reddit, so this will suffice as the next best time killer I guess?
I have never felt the need to own a portable backpacking chair, and consider my breaks to always be very comfortable in general when they are spent on the ground. No feeling sore or stiff or uncomfortable... well, ever, that I can recall.