Why Thermal noise power is P = kTB, where k is Boltzmann's constant and T is absolute temperature. At room temperature (290 K) and a 1 Hz bandwidth, that works out to about -174 dBm, which is the absolute floor a perfect, noiseless receiver would see at its input. Any real receiver adds its own noise on top of this, and the amount it adds is its noise figure. Widening the bandwidth raises the floor by 10 log(B), so 500 Hz of CW bandwidth sits about 27 dB higher, near -147 dBm.
Watch out The choices citing 3 dB or 6 dB above theoretical minimum describe noise figure, the excess noise a real receiver contributes, not the reference floor itself. -174 dBm is the reference point, not an offset from one.
-174 dBm = kTB at 290 K in 1 Hz. Add 10 log(bandwidth) to get the floor in a real filter.