At a high level, there's always a period of initial heating followed by a period of stable temperature. To be completely precise, the stable temperature is reached asymptotically, but in practice we think of it as preheating til close enough, then stable. When food is added, the temperature drops then recovers.
We tend to cook at a relatively stable temperature, especially for more sustained cooking, though there are certainly exceptions, where the goal is simply to reach a given temperature, and then you're done cooking. I'm not sure if these are what you're looking for, but:
cooking meat - usually you want to reach a certain temperature for safety/doneness, and not overcook.
candymaking - the goal here is to carefully heat sugar to a given temperature, and then stop as soon as you reach it.
thickened sauces, puddings - starch and egg just need to get heated enough to set.
In your slow cooker example, yes, in equilibrium the temperature will be kept pretty much at 100C, since there should be at least a bit of liquid inside and it'll be at a very low boil. But it does take quite a long time to reach that equilibrium starting from room- or refrigerator-temperature food; sometimes people will even heat on the stove then transfer to the slow cooker.
It's pretty much the same on the stove. A given setting on the stove will translate into some amount of power. (You may not know what that translation is, but the stove is still producing some fixed power.) In combination with whatever's sitting on top of that burner, that'll result in some equilibrium temperature, where the power going into your pot or pan is equal to the power it's releasing into the air around it.
If you're cooking a pot of something with liquid, it'll take time to heat up, but once in equilibrium you'll presumably be simmering or boiling at a pretty constant temperature. If the liquid is thick or if things are stuck to the bottom, the temperature may not be quite constant through the whole pot, but it's certainly not just getting hotter and hotter over time.
If you're cooking without liquid, you generally find a setting that keeps things at roughly the temperature you want, and you generally preheat the pot or pan. You don't want it to take forever to cook, and maybe you want some browning but you don't want it to burn, so you get the heat in the right range. Again, as you add things the temperature will drop and recover, but you definitely don't want it getting arbitrarily hot and burning your food. So outside of preheating and recovery, the temperature will be roughly constant.
Even if you put an empty pan on the stove, and even if you crank the stove all the way up, it'll eventually reach some equilibrium temperature. It may have ruined your pan by that point, though.