Welcome!

The father of Warhammer 40k art direction and the man that has inspired me as an artist, down to inspiring my current artstyle (and I am sure will continue to inspire me, even in death). 40k just wouldn't be 40k had it not been for the foundation of grimdark sci-fi that he laid.

Farewell, you absolute legend.

It's the latter.

It's not entirely accurate to say that the Emperor is physically a living human person. Nor is he only a manifestation of immense psychic energy that pretends to be a walking/talking human. He's something in the middle, and his real physical form is not exactly what we generally consider to be his general pre-internment appearance. (the long dark hair, the intense stare, the dark skin, etc)

The Emperor can choose to look however he wants. Over the course of human history, the Emperor has directed, and sometimes tried to control the development of our species. He has sometimes become historical figures that gained their own myths and legends, like Alexander the Great and Saint George. Other times, he kept to the shadows and influenced other people to do extraordinary things in the name of human progress and domination of the galaxy. Every time, he took on a different name, a different face and lived a different life. In older canon, he even explicitly had children over the millennia who discovered they were immortal. (but this has been changed/retconned/altered so many times)

In the story "The Last Church" we get a very literal glimpse at the Emperor in both his true form and a physical body he chooses to use when interacting with people. They are VERY different, and his real form is almost incomprehensible to an average mortal mind. The sheer power of the Emperor's living presence is overwhelming, and his psychic might is something the galaxy has not seen in millions of years.

The Emperor has no need for mechanical augmentations because his power is so much more than any mere human-made machine. Anything he uses as a tool is often made by his own hand, and imbued with power and magic that can never be replicated, such as his sword. Which is still being used in the current canon and has burned a significant part of Nurgle's garden.