Near home, carry one spare tube on my bike. When touring, carry two tubes plus a self adhesive patch kit. I find that the self adhesive patches only last months, but months is good enough for a tour, I can glue a patch on at home.
Like Dan, I also have a pair of medical gloves with my spare tube. Pre-Covid, it was easy to ask at the dentist or doctor office for a few spare pairs for bike repairs. But now that stuff is valuable, I do not even ask now.
I also keep a piece of Tyvek cut from a postage envelope in case I need a tire boot. Never used one yet, but maybe my preparation means I never will?
Keep my tube clean in a plastic bag, I do not want a bit of grit like a sand particle to stick to it and get inside my tire. If that happens, that will be the next flat. When at home, I wash off the wheel and tire to remove all grit before removing a tire, but that is impractical on a roadside repair, but I still take great pains to keep any grit out of my tire and tube interface. Back in the days when trucks used inner tubes, I was replacing a wheel on my truck in the middle of the night because the mechanic that had changed my tire a few hundred miles before let some grit get into the tire. I mention this because I have seen mountain bikers carry a bare tube where dirt and mud can get all over it.
I do not mark my tube and tire for reference, but I always put the valve stem centered at the tire label. That way if I find where the puncture in the tube is, there are only two possible places that I need to inspect the tire.
I do not use tubeless and have no plans to do so. I average about one flat a year. But I ride half a dozen different bikes in a year, I would rather deal with one flat than have to deal with sealant in half a dozen tires for annual maintenance.
I had a flat about five years ago, the tube suddenly started leaking and the puncture was extremely close to the valve stem, so close it could not be patched. I used to use metal presta to shrader adapters to use presta tubes in shrader rims. And the metal apparantly rubbed on the tube. On all my rims I replaced those with the Mavic plastic ones. And just to make sure, I have cut small squares of rubber from an inner tube, punched a small hole in that square and stretched that over the valve stem. Now there is a piece of sacrificial rubber between my rim and tube at the valve.
https://www.sjscycles.co.uk/rims-tape/mavic-rim-valve-hole-drilling-converter-insert-schraeder-to-presta/I once commented that Lezyne pumps that thread on and off the presta valve stem on tubes with removable stems, sometimes the valve stem stays in the chuck when you remove the valve stem. Dan had mentioned Loctite, which I now use but I am extremely careful to make sure it is only on the threaded part of the valve core and none of that is on the valve face. Plus wrenched tight.
A friend of mine lost a presta valve stem, it shot out of the valve stem under pressure. It apparently was not threaded in tight. That is another reason to carry a spare tube.