For Linux: It render the framebuffer directly -- you can check the code GuiLiteSamples/HelloParticle/BuildLinux/main.cpp

For MCU: It call LCD driver for rendering pixel -- you can check the code GuiLiteSamples/HelloParticle/BuildSTM32F103/main.c

For Qt, MFC, iOS, Mac: It build UI as a memory bitmap, and map it on picture Widget of Qt/MFC/iOS/MacCocoa