In my experience the recruiters are great about helping you prep. They have a PDF called "Google Interview Prep Guide Site Reliability Engineer" that talks about SRE-SE vs. SRE-SWE vs. SWE, and links to books and topics to study.

Kirk McKusick's "FreeBSD Kernel Internals" course[1] was recommended to me, and it was excellent. But not cheap at $1495.

[1] https://www.mckusick.com/courses/introdescrip.html