I've found it ok to understand maybe 70% of PKI, but that I always need to review that last 30% (OSCP stapling, cross signing certs, whatever the heck the formats are, different extensions, more that I'm forgetting).
I've put my favorite learning resources and tools into https://www.bbkane.com/blog/learn-ssl/
PKI is like a kitchen knife. The hard part is not getting one, or picking it up, it's using it without cutting yourself. But people treat the 'knife' like all aspects are magical.