Nice teardown https://www.allaboutcircuits.com/news/teardown-tuesday-hb100...

I'm sure they are cheaper, but I don't think they were ever really expensive. They have been used for automatic doors, like at the grocery store, for quite some time.

No, not really, doppler radar based proximity sensors (used in e.g. baths) cost about 40 € more than 10 years ago. It's not complicated technology.

I don't know what they're doing in a bath, but small radar systems have been used as proximity fuses for anti-aircraft shells since WWII.

As light switches and such; PIR sensors don't really work in a shower room, for example.

What's an e.g. bath?

e.g. is https://en.wiktionary.org/wiki/exempli_gratia

which you can pretty much read as "for example, [example(s)]"

My mnemonic is ‘eggs ample’. :)

Commonly confused with i.e. (id est), which trasnslates to ‘that is’. The mnemonic there for me is ‘in other words’.

Just assuming you were being earnest (perhaps learning english!) as there wasn't much to gain from interpreting it another way.

There actually isn’t much to these tbh, oscillator, mixer and pcb antennas. To use these in an application you would have to still process the analog waveform coming out. Fortunately the frequency is very low (audio range) allowing for all manner of inexpensive detection/measurement circuits.