About RF mode port

[zwy2025]

This is defined in the source code.
// Telemetry
#define TELEMETRY_DIR_GPIO GPIO_PIN(GPIOD, 4) // PD.04
#define TELEMETRY_TX_GPIO GPIO_PIN(GPIOD, 5) // PD.05
#define TELEMETRY_RX_GPIO GPIO_PIN(GPIOD, 6) // PD.06

// External Module
#define EXTERNAL_MODULE_PWR_ON() gpio_set(EXTMODULE_PWR_GPIO)
#define EXTERNAL_MODULE_PWR_OFF() gpio_clear(EXTMODULE_PWR_GPIO)
#define IS_EXTERNAL_MODULE_ON() gpio_read(EXTMODULE_PWR_GPIO)
#define EXTMODULE_TX_GPIO GPIO_PIN(GPIOC, 6) // PC.06
#define EXTMODULE_RX_GPIO GPIO_PIN(GPIOC, 7) // PC.07

// Internal Module
#define INTMODULE_TX_GPIO GPIO_PIN(GPIOB, 6) // PB.06
#define INTMODULE_RX_GPIO GPIO_PIN(GPIOB, 7) // PB.07

Here are the translations of your questions:

1. Why do we choose to enable the Internal RF Mode on the screen , and in this case, the serial port uses Internal Module port(a dual-wire) communication? Currently, most RF modules use single-wire communication. Are there any dual-wire communication RF modules?
2. When selecting External RF Module on the screen, it uses the Telemetry-defined interface instead of the External Module interface. Why is that?
3. What kind of modules are connected through the External Module interface?

[pfeerick]

Why do we choose to enable the Internal RF Mode on the screen , and in this case, the serial port uses Internal Module port(a dual-wire) communication? Currently, most RF modules use single-wire communication. Are there any dual-wire communication RF modules?

Incorrect. Most (but not all) external modules are half-duplex (single wire), but internal modules are mostly duplex (two wire). i.e. ELRS, AFHDS3, and probably the 4IN1 MPM. Some external modules can also be duplex, which is why they don't work with all handsets. (i.e. ACCESS modules).

When selecting External RF Module on the screen, it uses the Telemetry-defined interface instead of the External Module interface. Why is that?

I'd need to look through the code properly to be certain, but this is probably because this is related to module telemetry (between the handset and the external module, not over-the-air (OTA) telemetry).

What kind of modules are connected through the External Module interface?

Literally that... any nano or JR form factor RF module.