It's all bytes but untrusted user data is stored in memory pages which are not marked executable.

The CPU physically will not run instructions which are in areas of memory which are not marked as executable. This is a foundational principal of computing security.

> In computer security, executable-space protection marks memory regions as non-executable, such that an attempt to execute machine code in these regions will cause an exception. It relies on hardware features such as the NX bit (no-execute bit), or on software emulation when hardware support is unavailable. Software emulation often introduces a performance cost, or overhead (extra processing time or resources), while hardware-based NX bit implementations have no measurable performance impact.

https://en.wikipedia.org/wiki/Executable-space_protection