Huh? Usually (using any of the `embedded_hal` crates) it'll be something like `let mut gpioa = dp.GPIOA.split();` to get the GPIO A port from the Device Periph

Huh? Usually (using any of the `embedded_hal` crates) it'll be something like `let mut gpioa = dp.GPIOA.split();` to get the GPIO A port from the Device Peripherals list `dp`. `let pa5 = gpioa.pa5.into_push_pull_output();` to declare which pin in that GPIO port you're using, and set it into a push/pull output (input pull up, input pull down, input floating, and output open drain are of course also defined) and then `pa5.set_high();` to set it high (`pa5.set_low()` for low). There are HAL modules for most common MCUs, and a few uncommon MCUs. Use the HAL. The official embedded Rust book also includes how to write such a HAL, which seems to be the bit you stumbled on. Ignore it if you're not writing a new HAL crate.