In a normal ICE car your engine operating temperature doesn't fluctuate much unless you've been lazy on maintenance for several years and your hoses are cracked and there's leaks. A new engine won't wildly swing in temperature and will maintain temperature in cold or hot environments.
Not to quibble too much(or detract from your point-EVs in general, and Teslas specifically, have a cooling system too), but the temperature even of a new car varies more than you might realize.
It is desirable at least to a certain extent to run an IC engine as hot as possible without actually causing damage to the engine. I'll add a side-note onto that in that there can be quite a bit of variation at different point in the engine-the exhaust valves and rod bearing shells are often the hottest parts of the engine, while paradoxically the intake valve runners(which are usually very near the exhaust valves) can be the coolest part of the engine. Also, even with higher thermodynamic efficiency, higher cylinder temperatures can be counter productive for emissions since they tend to promote NOx production(generally a bad thing). Air cooled engines tend to both run hotter and have higher cylinder temperatures than water cooled engines, and if I'm not mistaken emissions regulations were sort of a final nail in the coffin for Porsche on air cooled 911s.
It turns out that a water cooled engine tends to like to run with a water temperature in the 200-220ºF range-this balances efficiency, mechanic integrity, and NOx production. Coolant thermostats are a somewhat standardized part(in that most are of the same general design and shape, and really only vary in their diameter-in an unrelated side-note I can probably walk into any parts store in the US and find a thermostat for my MG in stock because it's the same diameter and thus interchangeable with a small block Chevy), but typically come with a couple of different temperature ratings. In the ancient past, you might see a 160º thermostat, although this is now exceptionally uncommon(I had an old time hot rodder tell me that he used to run 160º tstats in his flatheads Fords in the winter because he'd use alcohol as an antifreeze). From probably the 1950s-1980s, 180º was normal, while 195º is essentially the universal standard now. On the "hotter is better" principle, I usually put 195º even in old cars unless I have a good reason to do otherwise.
When everything is operating properly and absent some external conditions which I'll touch on in a moment, once the engine is at operating temperature, the thermostat governs the
lower limit of operating temperature for an IC engine. For those who have never worked with one/changed one, in this context the thermostat is simply a valve that is placed such that when the water temperature is at the rated temperature of the thermostat, it opens and diverts water through the radiator. When the water temperature is below that, the thermostat closes and most of the coolant only circulates through the engine block. IDEALLY, the thermostat will hold the engine temperature close to its value, but again some conditions are outside that.
There are two fringe cases that are still quite common where the temperature generally varies-one above, and one below. The first is in hot weather in slow-moving or stop and go traffic. There are a couple of factors going on in raising engine temperature in these circumstances. The first and probably least significant is that in those circumstances the A/C compressor is often working hard and both dumping extra heat under the hood and putting an extra load on the engine. The second is that high ambient temperatures make the radiator less efficient at lowering the temperature of the coolant. The third is less significant now with electric cooling fans, but at idle or low speeds mechanical fans do not provide much air circulation(electric fans cool much better in these circumstances, but pull a LOT of current so again put somewhat more of a load on the engine). Along with this, at idle speeds, the water pump isn't moving as much water as at higher speeds, and in fact in extreme cases you can get localized boiling in the water jackets that dramatically reduces the heat transfer at those spots. The most significant point, though, is that there's an almost complete absence of "ram air", which is air that gets forced through the grille-at speed the amount of air moved by the fan is insignificant compared to the ram air, and again the temperature goes up. In a properly functioning cooling system, the temperature won't get high enough to cause damage, but it's still there.
The second common circumstance where temperatures can stray outside the ideal range is sustained moderate speed cruising(say 50-70mph) at lower temperatures. On flat ground and steady speed, there's not a lot of load on the engine at these speeds, and there is a LOT of ram air at relatively low temperatures cooling it off. TOO good of a cooling system can cause the engine to struggle to even keep the thermostat open. In some areas, it's common to see vehicles(especially semis) with part of the grill or radiator blocked off in the winter to minimize this effect-there are nice covers made to do this, but the cheap way is with a piece of cardboard. In the MG, which has a directly driven mechanical fan(no clutch), overcooling is a real problem in the winter, and I actually block the oil radiator(not the main water radiator) completely with a purpose made blanking plate. The louvers in the grille mentioned above do this automatically, although this isn't exactly a new concept. I've seen 1920s and 1930s vehicles that had the same, although manually operated.
If you hook up an OBDII reader and drive around, you can usually monitor the engine temperature in real time. The temperature gauge on the dash on most modern vehicles is useless to this and I've often called it a "glorified idiot light." The reason for this is that basically, the gauge will "creep" up as the engine warms up until it gets to the center. At that point, the ECM will generally lock the needle there for operation. At least in vehicles I've had, if you see the needle move past N, it's often already too late. There are basically two positions past N-almost all the way over at "H", which means "things are getting dangerously hot" and then pegged which is usually accompanied by the engine going into limp-home mode. I saw both of those positions one time in a previous car when I had an almost impossible to track down bubble in the cooling system that would cause falsely high gauge readings.