How to create an OS in C (For practice)?

Question

C++ / C#

I did it on asma, but it turned out to be a difficult boot file with not great opportunities, so I think it might be easier in C

Answer 1

In C it will be the same as in asma, since your approach is incorrect.
In addition, C will impose on you the use of libraries, and since the OS is its own, there will be no libraries and you will have to insert a bunch of inserts in assembler.
Almost all modern operating systems in their bootloader implement the simplest support for the file system so that you can already load the main kernel. True, BIOS interrupts are actively used.
In general, take and disassemble GRUB, the sources are open - https://www.gnu.org/software/grub/grub-download.html