Electronic – I know basic gates but not their structure

Possible reasons:

1. Load Balancing
   • The driver of A has an unknown number of fan-out to drive. Fan-out within the circuit and the parasitic it induces can be calculated for the specific circuits, but we do not know the other circuits that are connected the driver. Essentially the inverters are being used as buffer equivalent. and help manage the parasitic.
2. Timing and total current
   • To reduce the transition glitch, the second state inverters can be sized for a faster transition switch. Doing so makes the NAND gates input update near the same time. With the inputs changing less periodically, power can be saved and transition glitches can be reduced.
3. Signal boosting and power
   • Lets say VDD = 1.2V but the input is 0.9V. The input is still a logical 1, but considered weak which causes slower switching and burns more power. The first inverters can be sized to handle transitions better, making the voltage more predictable for the rest of the design.
   • There is also a possibility of the change in the voltage domain. In this case the inverters in the first state can act as a step down, e.g. a 5V input domain to 2V domain.
4. Any combination of the above

This is down to Boolean algebra rather than electronics. We have for your initial circuit with inputs \$A\$ and \$B\$ and output \$Q\$:

$$Q=\overline{\overline{A\land B}}=A\land B$$

Where \$\land\$ represents AND and \$\overline{\cdot}\$ represents NOT. So in this case you have created an AND gate. For the second circuit we have (where \$\lor\$ represents OR):

$$Q=\overline{\overline{A}\land\overline{B}}=A\lor B$$

By De-Morgan's laws. Therefore it is a functional OR gate.

Notice that the crucial difference between both of your expressions is the first one is negating the entirety of the NAND output whereas the second one negates each of the inputs before NANDing them.