first they had machine code
then they wrote the assembler in machine code
then they rewrote the assembler in assembly
and assembled it with the machine code assembler
then they wrote the C compiler in assembly
and assembled it with the assembly assembler
then they rewrote the C compiler in C
and compiled it with the assembly C compiler
then they had the C compiler
and everything else was written in C

(note: this is a massive oversimplification and ignores much of the history of programming languages, but it at least gets across the idea of how bootstrapping is done)