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.