Entering other countries via car

Yes, thats right. Shengen is open to travel with no passport checks. There are occasional restrictions due to high level of illegal immigration. 

True, once you are in the Schengen area practically there is little to prevent anyone doing as you say, the problem comes however when you leave as your passport will have been stamped on entry and if the date of your departure is more than 90 days later then you will be branded an overstayer and potentially banned from entering the Schengen area for up to 3 years.

As Darro says, the 90 days is in Schengen in total. Fly to France and you are in the Schengen area so are free to go to any other Schengen country with no passport or even border checks. But that 90 days is valid for all of Schengen so, even if you left from say Italy, the date you entered France would count for the 90 days even if you depart from another country. 

And you would have to show your passport on entering France and on departing Italy.

I agree and I only went to 4 countries. Wonder what would happen though just for example, you drove from a European country or a Schengen country to a non European/Schengen country and flew back from there? Because in theory (according to the passport) you're still in Europe? Or would that not work? Its a possible loophole for people or a boobytrap

They should still check though because not everyone is entitled to be in that country or they may have already overstayed and can continue to do so because border control hasn't checked their details 

When you drive from a Schengen country into a non-Schengen country you passport will (or should) be stamped, as you are exiting the Schengen area. Same as flying or travelling by boat. So say for example you drive to the UK, stamped out at Calais, fly back to Alicante, stamped back in at the airport.

 Being in the schengen zone gives the right to travel around a borderfree zone of EU schengen countries, at least in theory.

Rgds,

Aitch.

A little light reading for anyone who is interested:

https://home-affairs.ec.europa.eu/policies/schengen-borders-and-visa/schengen-area_en

Ah, so you would still get done when leaving.