One approach is to pull the tree out of its bridged position. Use a tractor (best), a pickup (usually works), or a come-along if that's all you've got, although it's slow going. Regardless of what you pull with, if the tree doesn't budge you may have to carve off some bits to reduce its weight or friction-producing capacity. Look especially to remove those limbs that are locked up against something. You'll have to use chain or good strong rope for pulling (or wire rope if you have it), and be careful not to use under-capacity line that could break and recoil toward you.
If pulling is not within your bag of tricks, begin cutting in place. Be aware that if you just cut down through, top to bottom, you'll soon have your saw pinched. So best to cut a little ways in from the top, say 20--25% of diameter, being very aware of how the tree reacts so you don't get pinched. Then remove your saw and cut up from the underside to meet the top cut. At every stage pay strict attention to what the tree does, because especially with dead wood (which is less elastic) something can give with little notice. You don't want to have legs or feet or whatever in the wrong place when the thing pops loose and drops.
The tree in the first two photos--your first move is probably to sever it from its attachment to the stump. Study that thing beforehand to determine where gravity and other factors will pull/push it, and take care not to be in the line of fire.
Most of this stuff is best learned from experience, the kind that can't be imparted thru the internet. Take your time and think about each cut and its likely results. It's the folks who just tear into it without thought who most often entertain us on YouTube.
Hope this is helpful.