Can carmakers chips be emulated by more available ARM microcontrollers?

That really depends upon what the "chips" the carmakers are using do.

If they are using highly integrated micro-controllers similar to these (this is but one example of this family) https://ww1.microchip.com/downloads/en/DeviceDoc/40300C.pdf then to "emulate" this chip with an ARM CPU involves also designing external hardware to interface with the ARM chip to recreate all the built in interfaces of this one chip (i.e., the analog comparators or the PWM controller or the serial UART). Other chips in this family have an even larger complement of integrated interface hardware.

Changing to a new CPU does not always involve just writing new software, esp. in device controllers it also involves recreating all the hardware necessary to interface that new CPU with the existing hardware that the old chip was controlling. And recreating all that interface hardware requires a redesign and a retest of that redesigned hardware. Both of which can take significant time.

Not really, no. A lot of the problem is that automotive circuitry is a real messy place, so you need a lot of high tolerances to things like voltage spikes, background noise, temperature, etc. You also have to consider that most chips have different pinouts, with different on-chip components, usually with some analog components, so would need to find one that is compatible, or spend time and money developing, documenting, and certifying, a board revision, test to make sure that it is a drop-in replacement for things like warranty repairs. All of this takes a lot of time and a lot of money to do.

According to [0][1], car makers are unwilling or unable to switch from older Intel chips to modern (smaller process) Intel ones. Switching architectures doesn't sound more feasible.

[0]: https://jalopnik.com/i-asked-experts-why-carmakers-cant-just...

[1]: https://news.ycombinator.com/item?id=28728965

Can those take the heat and run on the power budget? Cars get very hot when left in a sunny parking lot at noon near the equator. They get cold when left overnight near the poles. Both are regular things.

The car might have a 12 volt battery, but whe. The starter is running you are lucky to get 2 volts, and there are constant spikes. While this is mostly about the power supply, you need to handle whatever the power supply gives, which might not be good if the original chip worked.