Why is the dB of the FFT plots exceeding 96dB for these signals sampled by an 16 bit ADC
The dynamic range of (about) 96dB means that the ratio of a full scale signal (FSD) to the total power of the quantisation noise is about 96dB. Or the total noise power is about -96dBFSD.

However, when the noise is analysed into discrete frequency bins by doing a DFT, that noise power is split among the many bins. Another way to consider it is that the noise bandwidth of the each frequency bin is rather less than the Nyquist bandwidth, so can contain less noise power than the entire bandwidth.

In fact one way to distinguish noise from spurious signal is to change the  DFT resolution bandwidth. Noise power will drop, coherent spurious signals will not.