Broadly speaking, there are four major architectures used for Linux (and other OSes): x86, x86_64 (AMD64), AArch64 (ARM64), and RISC-V.
x86
This is a 32-bit CPU designed by Intel, limiting RAM size to 4GB due to the maximum 32-bit memory address possible. These are often found in older machines, legacy systems, and some embedded devices.
x86_64/AMD64
In 1999, AMD introduces a 64-bit extension of x86, which as later adopted by Intel. This allows data process in 64-bit chunks (making the processing more efficient) and for much higher RAM addressing (to the tune of 16 exabytes).
This is the most commonly used architecture today.
AArch64/ARM64
This is a 64-bit version of ARM processors; they are usually used in mobile devices, cloud servers, and power saving centres.
RISC-V
This is an open-source CPU design that is used by companies and developers to customise processors for their own specific needs.