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First, the RX and TX signals are internally connected to the USB<=>UART adapter (a CH340 circuit in Arduino UNO chinese clones).
It is not feasible to connect the TX from the internal CH340 together with the TX from the HC-05. Luckily, the UART signal idle state is always high. So it is possible to connect two TX together with open collector drivers, provided that the two TX's don't emit together. A shottky diode is adequat to transform a totem-pole output into an open-collector one.
Second, the HC-05 logic signals have 3.3V levels (a priori not 5V-compatible) while the Arduino UNO generates 5V levels. A simple resistor was then inserted in the line from Arduino to HC-05. Combined with the ESD protection diodes at the input of the HC-05 clamps the signal to 3.3V.