embeddTrway690
Lot of apps use natively compiled libraries through NDK.
The problem was too many people entering engineering degrees after a temporary 'IT boom' decades ago, seeing that as a way towards upwards social mobility.
That's one reason I use linux. I am a student and I have this 4GB laptop I brought for less than INR 30,000, and linux is still super responsive on this. Windows lags for 5 seconds before pressing Windows key and search results showing up.
* C/C++
Learn C and it's standard libs (stdio etc..), if you haven't already. Choose a good book for this because many tutorials etc.. you find online are pretty incomplete. Then read one of your books about UNIX APIs.
Also it's worth learning how to use a C debugger (gdb or visual studio one).
* OS
You can look at linux source code but it would be daunting. I'd suggest starting with a Teaching OS and accompanying book, eg xv6 or minix.
Try doing modifications in it. toy OSes generally come with such exercises.
* Assembly language
Learning C doesn't give a perfect idea how machines work. Learn X86_64 assembly programming. you can inspect what assmebly output your C programs give using godbolt's compiler explorer website. Assembly is little boring so don't try to memorize instructions. The mental model is important thing.
* Basics of algorithms and data structures
Maybe you already know, because you worked in back end. But if you're not familiar with few data structures like hash tables, b trees etc.. it might be worth familiarising yourself.
Some miscellaneous topics you might get interested in: linkers / executable formats, OS level stuff related to computer networks, multi threading, file systems, SIMD / vector instructions in assembly.
As others said, write some code when learning. You don't need to do entire projects yourself, you can also play around with established projects.