What’s the same is I believe called the ‘advance width’—the width of the black part plus the side bearings.

The same width.

Sidebearings would definitely differ. For instance an O will have smaller sidebearings than an H.

Think of how things look when you type on a typewriter— you know, those noisy things with levers or a ball that slams the page each time you hit a key— I probably haven’t used one in 10 years, but I remember them! :-)

Every glyph in a true monospaced font will live in a space of the same width, and this space can be thought of as the equivalent of a cell in a grid in which every column is the same width. It doesn’t matter how wide or narrow the black is, or what the dimensions of the sidebearings are, at least by themselves. The key is that every glyph’s little “cell” is the same width (and, for all practical purposes, the same height, too).