The traces show the “power” of the signal (across all time points) for each frequency (or, rather, for a number of frequency bins).
This corresponds to the average energy of the measured signal for the respective frequencies.
It is not clear that this necessarily corresponds to neural signals. If, for example, your sensors picked up line noise, you will see a peak in the PSD plots around the line frequency, but this doesn’t mean that the participant’s brain produced this signal.
The “outlier” traces for subject 075 are likely faulty sensors. You can click on the traces to retrieve their name. You should inspect their signals very carefully and decide whether or not to exclude them from analysis. Same goes for this one (?) trace that oscillates somewhat for subject 076.
Nope, unless it is clear that the signals are epileptic related, then most likely you are just analyzing noise. PSD is just a summary way of looking at the data.
Analyzing epileptic iEEG signals is a fine balance between qualitative and quantitative analysis that requires a tight collaboration with clinical folks who actually look at the data and can tell you whether something is relevant or not. Or a good understanding of what typical neural signals “look like”.