There are several methods for interpolating additional frames if you want to slow down footage beyond its original frame rate.
First: duplicating frames. Pros: very fast, easy. Cons: low quality, causes stuttering, even with fairly small decreases in playback speed.
Second: cross fading between neighbouring frames. Basically each frame is dissolved into the next one. This will work well for a lot of footage down to say 1/3 of the original speed, after that it gets a bit obvious. Pros: fast, relatively easy, lots of software does it. Cons: limited capability.
Third: using motion estimation algorithms to create frames from neighbouring frames. Basically you apply a morph between neighbouring frames, with some intelligent tracking of pixel motion to create realistic motion. Pros: very good results with some footage, even with large slow-down. Cons: compute intensive, so slow to render. Not many apps do it, especially free/cheap ones. Can cause weird and very obvious artefacts when the tracking goes wrong–in fact, I've rarely been able to get this to give satisfactory results with real-world footage. If you have simple movement against a plain background, e.g. green-screen footage, it can work really well, but for any complex scene you generally get all sorts of really obvious weirdness.
The final techniques is still under development, and will probably keep getting better. AI is looking very promising as a tool for this kind of thing, so we may see some quality gains soon.
P.S. I'd say that Sony claim about interpolating 24fps to 960fps is 99.999% marketing bullshit.