SREs are very hard to hire, speaking from experience. At Google SRE directors and VPs will often cherry-pick promising candidates from the mainline SWE hiring pipeline and give them a "hero call" to convert them to SREs. SREs at Google are also paid more, controlling for level and performance, as a way to hire and retain.
In all seriousness they make it out to be more than it is. From my experience going through their hiring pipeline there seem to be two tracks in SRE; software and sysadmin. If you score higher in algorithms and data-structures, presumably, you'll end up working more on tools and libraries whereas in the other you'll work more on infrastructure and automation. Either way both tracks work together on the same team towards the same goals.
If you want in be prepared to solve simple-to-tough algorithms problems and be quizzed on TCP re-transmission, Linux system calls, and memory pressure. It's a bit challenging because you not only have to know Big-O well enough to estimate the asymptotic complexity of an arbitrary algorithm but you might also be asked what a sequence of TCP packets would look like if you sent some data and pulled the plug or what the parameters are to a given system call on Linux. You quite literally have to know everything from how virtual memory works, how to implement a fast k-means, how the network stack works from top to bottom, etc, etc.
If you've done any work in cloud development and supporting moderately large one it's that but bigger. Make one a hero, it does not.
It's just a guy on the phone telling you how you won't be like those other chumps, you'll be a hero. The few and the proud. Standard recruitment techniques.
