Is a baseband signal single sided?
It depends on how you represent a signal and what information you have about a signal.

Look at your first two drawings these only have positive frequencies. To make these signals double-sideband there would be negative frequencies !

What is a negative frequency ? Of course in reality all frequencies are positive but one can mathematically represent a negative frequency when you have information about the phase of that signal.

But if you only have a single wire with a signal (and a ground but let's ignore that for the moment) then how can you know what phase this signal has ?

The answer is that you can't. Phase is always relative so you need some reference or another signal to which you can compare the phase and know the phase. Actually the phase difference is all you need .

So you need two signals to make those negative frequencies. This a called a quadrature signal, often named I and Q. Read this for a more thorough explanation.

If you would generate these I and Q signals you would have a spectrum containing positive and negative frequency signals !

Now when you want to wirelessly transmit such a signal you would multiply it with an RF (Radio frequency) carrier. The result of this is a spectrum with your original baseband signal folded around that RF carrier frequency, like in the 3rd drawing in your question.

This is because the multiplication (of BB and RF) results in sum and difference frequencies. If you have a BB spectrum with only positive frequencies (so no I and Q) you'd get a symmetrical spectrum around the RF carrier.

If you however did have a BB spectrum with positive and negative frequencies this positive frequencies and you mixed that in the proper way with a quadrature mixer then the negative BB frequencies would end up on one side of the carrier and the positive frequencies on the other side.

How is this usefull ? Well, having the same spectrum at either side of the carrier is not efficient, you're sending the same information twice. Or actually at the same time but at two different frequencies. Instead you could make those sides different, even transmit different signals (or no signal in SSB (Single Sideband)) and make optimum use of the available spectrum. Wireless systems like Wifi, GSM, UMTS they all do this.

Wifi uses a BB signal that can be as complex as 64bit QAM, see here. The bits to transfer can be represented in a constellation diagram, note that this contains phase and amplitude information. The spectrum of such a QAM signal does contain negative frequencies so it is essential to process it in a quadrature capable transceiver. If you didn't cater for those negative frequencies, this QAM signal would be completely destroyed.