In those cases it's not used to make detection harder though but to be more resilient against interference.
Fun fact, the famous actress Hedy Lamarr actually was instrumental in inventing it!
With frequency hopping only one frequency is used at once, and they're selected according to a sequence that matches a shared PRNG. As I understand it, that's what Bluetooth and some Wifi links do. They switch between discrete bands very fast.
Spread spectrum is a little different. During modulation, as with FM a series of side bands spread out. Normally we would limit these as they're considered "interference". But with the right modulator you can spread (and indeed encrypt at the RF link level) information into some bands but not others. Although all sidebands are present, to an observer who can't tell which ones carry the data at any moment, it makes no sense to even try demodulating the signal.
Neither solutions are much use if you need to communicate back because triangulation alone is enough to find you.
Curious if this tallys with your understanding, its over a decade since I had anything to so with this sort of thing. The best summary I recall was in Ross Anderson's "Security Engineering" book (first ed.)
Spread spectrum refers to any technique where a narrowband signal is deliberately spread to occupy a larger bandwidth.
Frequency-hopping is an example of a spread spectrum technique; they're not different things. Direct-sequence spread spectrum (which is what I think you're describing) is also a spread spectrum technique.
There are also other techniques; the most popular one is probably the chirp spread spectrum as used in LoRA.
If I understand the capabilities of modern Software Defined Radio systems, then they can monitor many different frequencies at the same time which might defeat those old systems. It’s been a long time since I read up on the current ideas and capabilities.
Haven't been keeping up with it as much as I should, but that's very interesting.