It was not very common to interleave assembly in MS-DOS IDEs. Assembler and its IDE were separate tools you paid for. But not unheard of.

You could "dump" your OBJ file for assembly.

Later C compilers got some better inline assembler support but this was towards the 32-bit era already.

Also Borland had its own compiler, linker and such as separate binaries you could run with a Makefile but you really never had to, as why would you when you can do that in the IDE in a single keypress.

It was quite common actually, most DOS C compilers supported asm{} blocks and Turbo Pascal also supported inline assembly. Paid assemblers like MASM were high-end tools.

On Unix though it was more common to have .s files separately.