The bootstrap header file is <avr/io.h>. That header file includes a processor specific header file from the same directory.
Things like SPDR and SPSR are macros that expand to reference specific memory addresses. In the general case, the compiler simply generates code to access the memory location. There are at least two specific optimizations the compiler can perform. If the memory location is low enough then I/O machine instructions are used instead of memory access machine instructions. If the C expression references a single bit and the memory location is low enough then bit manipulation machine instructions are used.