Decrypting messages for dummies

Thank you a lot, you have really clarified some basic facts. For this purpose is LoraEncoder function - for example to change measured value to number representation of this value, isn’t it?
To be sure that this is the right way: way from measured Temp=23.45 to 0x09 0x29 on the TTN Application data page with as small payload as it possible is (from Arduino):

float Temp = 23.45;
LoraEncoder encoder(buffer);
encoder.writeTemperature(Temp);
LMIC_setTxData2(1, buffer, strlen((char *)buffer), 0);

It is not clear to me how Decoder in Payload Function shows the right value for the temperature but the field in Data page showing me {“celcius”:105.05} in the same time. Did i done something wrong? Your help is really appreciated.

Thank you,
Roberto