IIRC the C64, like the BBC range of 6502-based Micros, had their BASIC in ROM and in fact booted to it in REPL mode by default. As such it was always in memory as the ROM would always be there¹. There were certain bits of the address space that were not safe to use if the user would drop back into BASIC as the ROM used those for its stack & heap and other scratch space, but otherwise you could ignore BASIC's existence once your machine code was running.
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[1] caveat: under usual circumstances, on the Beebs you could switch a different bank into the memory space used by the BASIC ROM, either another ROM or some “sideways RAM”
You can do the same bank switching on a C64. Through different bits of RAM location $0001 you can switch off the BASIC, KERNAL and character ROMs, exposing the RAM in those locations.
The C64 has a BASIC ROM and a kernal ROM. You can swap out the BASIC ROM or both ROMs for RAM using addresses 0 and 1 on the 6510.