The demo uses a simple prebuilt fixed-size font atlas texture and renders the entire UI (including character quads) via batched glDrawElements calls (one draw call per clip-rect).

But the way how text rendering is delegated to the user is quite flexible, microui basically calls these three user-provided functions:

    int r_get_text_height(void)
    int r_get_text_width(const char* text, int len);
    void r_draw_text(const char* text, mu_Vec2 pos, mu_Color color);

...how r_draw_text() is implemented is then entirely up to you.

it does not, it basically delegate that part to lower level library for which you have to write the glue code for, there is an example for SDL.