How does my iPad charge from the adapter or battery?

Your iPad will suffer no ill effects through being connected to its Power Adapter for extended periods. Your iPad will automatically regulate and optimise charging - and will not overcharge the battery. 

When used in kiosk applications - where external power is connected for extended periods of time - the iPad may modify its charging strategy and limit battery charge to ~80% of maximum. This modified charging strategy is designed to reduce chemical ageing that might otherwise occur if 100% charge is maintained at all times.

Nominal 100% charging will be restored when a normal charge/discharge routine is restored.

More information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

Using your iPad while connected to its Power Adapter will potentially reduce the frequency that you need to recharge the iPad battery. As your iPad will accumulate fewer full-charge cycles over time, this will prolong the service lifespan of the battery.

As an observation, I habitually use my iPad at my desk - and when doing so, my iPad is always connected to its Power Adapter.

Your iPad is always using its battery - however, when connected to a suitable Power Adapter, the iPad will draw required power from the external power source while the battery itself is charging.

Is it like a MacBook?

An informative old Apple tech article on Mac notebook batteries that was sacrificed at the Altar of Minimalist Design stated that, if a Mac notebook computer was on its external power supply and encountered a task that was using a lot of energy, the computer could draw power from both the external power supply AND the internal battery.

I don't know if iPads are the same. It would not surprise me if they are, because that is pretty clever power management.