There is a signal processing onramp course you can take and it can give you a bit insight on the topic itself. There are lots of things to consider and it all depends on the system you measured yourself.
pwelch does windowing, overlapping and averaging itself (if you do it with default values) and this looks already like a decent result itself that you can use to get the information you need.
You should have some values that you have calculated yourself so you can look for them in the graph. For example, these 4 peaks from 3 - 5 * 10e7 Hz seem like they are harmonics of some natural frequency of the system. Maybe, due to pwelch settings, you can't see them in the frequencies 1.3 - 3 * 10e7 Hz.
These peaks at 3.1 and 6.2 * 10e7 Hz, could also be harmonics of the 0.2 * 10e^7 Hz.
It's you who has to interpret the results and match them to your findings.
Maybe someone else can chip in with more knowledge, I would like to hear more opinions.
