OSDev.org

I have some experience working with bare metal embedded programming: all of my recent projects on Cortex-M chips uses a startup package I constructed myself using C and linker scripts, compiles with either ARM GCC Embedded toolchain or LLVM/clang supplied with Xcode (I work on macOS,) and uses the version newlib included with ARM GCC Embedded toolchain as C standard library.

Will those experience help me when creating a hobby OS for x86 (using GRUB2-on-UEFI Multiboot as bootloader) or ARMv7A (using u-boot as bootloader?)

They *might*. Having experience with low-level programming in general is pretty useful. However x86 is quite different than ARM.

Cheers,
bzt