I like this explanation, but the first paragraph mixes up things. ADCs (and sometimes DACs) are parts of most micro-controllers, but those are never used for digital communication. The most low-level primitive is GPIO block (general purpose input/output). This block can be found on most of chip pins, and it does the conversion between analog voltage and digital bit (as well as value latching and some other functions). With just them and the CPU you could manually receive and send digital bits and build complete communication protocols in software.

Also even if your STM32 has ethernet support, you still need a PHY transceiver that accesses the physical medium (wire or optical bus). This project implements both inside the Pico and requires just 3 external resistors. Very impressive feat of engineering, but not something you'd deliver as a field product.