Questions about Ugreen CM106

You can turn it off with the main button. I plugged it into the power sockets of the amplifier, when I turn on the amplifier it automatically turns on the bluetooth device as well, same thing when I turn off the system it also turns off the bluetooth receiver

The receiver is quite good for the price and the built-in chip is even very good. However, the output level of the analogue outputs is not too high. The device can memorise 5 players, but only one may be active at a time, i.e. you have to switch off Bluetooth on the others so that the device is not blocked. Personally, I would not recommend the device for a new "high-end" system but only to "upgrade" an old one. I have used it myself for a home-made boom box. However, if you're also planning to buy an amplifier, I would personally prefer to spend a little more money and go for the Rotel A11, for example, which has Bluetooth built in right from the start. Or then choose a setup where the entire Bluetooth -> amplifier signal path remains digital (e.g. via SP/DIF connections) - the analogue side of such devices is usually their weak point.

No. It is only a receiver. But there are devices that can do both. Or only transmit.