This section explains some questions that come up over and over again.
The USB-C port on the X10/X10s has a serious design flaw. Do not use it for anything other than charging with the supplied charger
USB-C has been designed to make it safe to plug arbitrary devices together without having to fear damaging any device this way. This done by mainly two main things that devices with an USB-C port must comply with. Firstly, never switch on power unless being signalled to do so and second never output anything other than 5V unless the devices agree with the USB-PD protocol and only by use of the USB-PD protocol to a different voltage. The signalling of allowing 5V output can also be by use of a legacy cable (e.g. USB-C to micro USB or USB-C to USB-C socket adapter).
The FrSky X10 the USB-C is abused as connector for the Lipo charger. It violates the both of the USB-C safeguards. Instead of presenting only 5V when signalled to do so, the USB-C connector on the X10/X10s directly connects to the battery and outputs battery voltage (up to 8.4V for the default 2S Lipo) regardless of what the other device is. As almost all USB devices are only specified and tested for 5V, connecting the X10/X10s to any other USB devices (via USB-C to USB-C or USB-C to USB-A cable) has a good chance to permanently damage the other device. The USB-C port also provides no USB functionality.
The (less common) mini USB connector on the X10 is fine and is the only USB that can and should be used to connect the X10 to a PC.
Note that the voltage on the external RF module PPM output pin is driven by the battery voltage on the FrSky radios. Especially for SBUS, which is normally 3.3V, this is too high. However, the output is current limited by a 10 kΩ resistor, which limits the current to around 1mA. The input protection diodes of FC processors should be handle the higher voltage at such a low current and connecting the output directly to a flight controller should be relatively safe. But it is still outside the specification and you are doing that on your risk. If you want to be really safe use a voltage divider or a level shifter.
This is a bit more exotic question but comes up often enough so that it needs a proper explanation why this problematic.
Smart.port (S.port for short) is a bus protocol that is used for connecting sensors/flight controllers to receivers. When two devices try to talk at the same time on the bus, the two signal interfere which each other and the data is lost. This problem is solved by having only one device on the bus that is the master. The receiver will periodically ask each sensor to send new data and when the sensor has its chance to send data no other sensor is allowed to talk.
On the Frsky radios the S.port pin between the internal and external module is shared, they are electrically connected. Unlike the receiver and its sensors there is no coordination between the two modules. That means if both modules send telemetry at the same time, neither gets through.
If you have little telemetry coming from both modules, you might be lucky enough that most telemetry sending is not at the same time and that most of the telemetry is send without errors. As soon as the amount of telemetry increases the chances of errors increase. They system might like it is working, giving you a false sense that everything is okay.
Even binding a receiver without telemetry is not enough to stop a XJT module from sending telemetry, as there some module status messages that are constantly sent. The only way to ensure that no telemetry is coming from an external XJT is though setting the DIP switches.
For the R9M module, enabling/disabling telemetry can be done by software. OpenTX will automatically disable telemetry if internal module is on too, to avoid these hard to detect errors.
The Crossfire module uses the sport pin not only for telemetry but also for input. If the internal module also sends telemetry, the communication to the Crossfire module is problematic.
OpenTX 2.2.1 and later allows you to see the telemetry receive error statistics under the statistics view (where also other debug information are presented).