So, classic foley involves having a load of sound-creating objects - paper to rustle, celery sticks to chomp and snap, and recording them all in time to video
Then a more modern way would be searching through soundcloud for the exact right sound
But im guessing the reality is a mix of the two.
And thats just the start. We now have to place the sound as convincingly coming from inside a cave or from the flat (apartment) above us.
Foley artists make sounds with the “real thing” for every movie and they “act”…
Foley mixer “spice up” those sounds for the exact purpos…
… of course there are also special sounds recorded for further use…
or sometimes not the real thing, if the ‘sound’ is ‘better’. i once did an audio commercial, where we needed the sound of meat hitting a table… needed to sound ‘big,’ like a brisket or something. so i set up a couple of mics, wet the floor, took off my shirt and flopped my back into the wet spot. looked weird as heck, i’ll grant, but the sound was spot on for the client.
since there’s often a timecrunch involved in foley studios, only so much time can be used for hunting the 'net for that exact right sound, though in down times mayhaps a library of “that sounds useful” might be built up, stored locally, and relabeled to the foley house specs. licensed libraries are another source.
for coming from inside a cave, a good reverb can get you far. i’d probably look at open source ir convolution type reverb, and grab some ir’s of various caves … find one that sounds ‘right’ to you in that use case. for the flat upstairs… well, two ways. record the sound from downstairs, or heavy use of EQ might be able to get the sound your after.
general advice: if you are doing the recording, learn your mics. placement, how to use them, how to manipulate them to get different effects. for example, muffling them with different materials, pointing them in odd directions (sound direct? or reflected off this cake tin… how does that change the sound…)
same goes for software, learn what all the effects in it do, how they manipulate the sound. the more you know this, the faster you can work.
and really important: keep your ears open. listen. and i don’t mean just while doing this. what sound does your blinker make when you turn it on. does a lightswitch in your house sound different than the others. what could that sound be? is that a lightswitch? or an alien beetle thing communicating? layer those sounds together… what if you combine it with a slowed/pitched down blinker sound. a ‘silly’ example, but extrapolate from there.
in the end, this is no different than visual artists, walking around tearing apart what they see in their mind on how to recreate everyday objects, and what they could also be if viewed from another perspective.