In Model Flying Geek mode...
In the "old" world it was 35Mhz that was dedicated to airbourne models (Plane, Heli's and Gliders)
While historic, 27Mhz was originally for general RC use but later became the frequency of choice for surface use (ie cars) , and 40Mhz could be used for any surface model but tended to be used for marine (ie boats and subs). 27Mhz had 6 channels but later was expanded to 32, and 40 Mhz has 30 or so possible channels. 27Mhz originally used differently coloured flags to show frequency in use, but as the band was expanded a black flag and channel number was used, 40Mhz use a green flag and a channel number and 35Mhz used a Orange flag with a channel number on it as there are 35 potentially useable channels on 35Mhz.
Almost all of these frequencies used an analog transmission mode, with no way to differentiate between transmitters. You CANNOT have two models active on the same frequency, as the radios will swamp each others signal, and the receivers not know which one is which. All model flying clubs had strict rules on who could turn on their transmitter at any one time, (as its obviously more serious in aircraft than cars if you lose the connection).
When we started to see cheaper Chinese toy models appear on the market (personal imports, or sellers not caring as they cashed in on a craze) we saw these toys being built for a global market and so saw any of the frequencies potentially in use, so you saw sometimes saw cars on 35Mhz, or planes on 27Mhz. You also say some models on 49Mhz, 72Mhz or 75Mhz, none of which are permitted in the UK, but that didn't stop them appearing.
There nothing wrong with 35Mhz, I still fly it, and its as safe as any other frequency, in fact potentially slightly safer as its dedicated to airborne models only, but its getting harder to buy new hardware for it as people have moved to the 2.4Ghz that uses a digital transmitting mode that can offer more features.
Now we are on 2.4Ghz many receivers have more electronics in them and most "bind" to the transmitter, so they should only responds to command from the transmitter they are bound to, think of it as akin listening for a serial number in the transmitted signal. As 2.4Ghz can be used for any model there's no need for makers to make three versions of a controller, one surface, one air and one marine, although some car transmitters are made with steering wheels as some people prefer them.
In theory as long as the receiver can "hear" the transmitter its bound to it should be OK, and will disregard any other transmitters signal. but there have been issues where the binding was programmed wrongly by the maker so it didn't work that way. 2.4Ghz systems usually do frequency hopping (some early ones did not) so interference should not be a big issue as it should hop away from the channel with interference, but if the entire band is jammed that will still be a problem. If there were jammers in use, they are probably blanketing the band as cellphones frequency hop too, and given frequency jammers are illegal they may not be that great on quality control so may be more powerful than needed, or poorly build and so may well bleed over to other frequencies than just cellphone ones.
Coat icon for very obvious reasons :)