They know when you leave because they stamp your passport on entry and exit because it applies to anywhere in the Schengen area, so they need a record of all your time within it.

If you did a lot of short journeys it would get quiet hard to tot up manually, so I don't know how they do that but I think there is a new system where you need a visa that you get online soon to enter the EU. Then it will probably be done electronically.

The 90 day rule, maybe a law that they won't bother enforcing, many laws are not always fully enforced. And if it were and you over stayed, what is the penalty? I guess if you broke the law in Tenerife and they charged you, then found you were over 90 days, they would also charge you with that. But if you kept out of trouble and were contributing to the economy, why would they care!
 I will be interested to see if anyone is ever in trouble if they overstay, and how would they know?
I know that in Monaco, to enjoy the tax free benefits you need to be there at least 6 months and a day each year, and I understand that they use the Postmen to report back any address's that appear to have long absences, then it is up to the individual to prove otherwise!

You are using a tiny little Island, who don't forget had a more restrictive lockdown than most other countries, I know I was there, as a comparison to other countries who have tens of millions.

And I am sure the people who have died prematurely no matter how old they were are glad you are not their GP or work in the nhs.

I didn't mention the hospital capacity in Italy because I don't know it. Probably better than ours

You can have a total lockdown of a single province of tens of millions in China because it is a totalitarian state and because the country is so huge they have the manpower from surrounding areas to deliver food to everyone, police them, knock up hospitals in a few days and staff them. They didn't lock down the whole county, just a tiny part of it and it was the epicentre of the pandemic, so not well spread out. But on the downside a hotel where they forcibly quarantined people collapsed killing hundreds and parents of a disabled child where forcibly removed leaving it to die.

Counties like USA and Brazil and India count there dead in millions because they have massive populations where millions die every year anyway. We count our dead in hundreds of thousands because 600,000 die every year.

You don't seem to be able to see large numbers in any perspective. There are 8 billion people in the world, so 60 million die every year, about the population of the UK. The number dying from covid is a tiny proportion and totally insignificant. Currently death rates in the UK are below average because a few people died a few months earlier than they would normally. Probably a year after the pandemic ends there will no net difference in the number of people that have died. In the main it finishes off people who have not long to live, whether they know it or not.

So yes I am totally against lockdowns that ruin life for the majority to save a tiny minority of unhealthy people. In Tenerife there are about 4 covid deaths a day, so they still announce their ages and if they have an existing health problem. They nearly always have a an existing health problem and / or are very old. When they are young without an existing problem it is major news and they have only been one or two out of a population of 1 nearly million.

Thanks for all the comments, as stated I was not trying to get around the 90/180 rule .
I just wanted to know how to visit LG and stay in my apartment as often as I did before Brexit.The sooner residents and visitors  arrive the better to keep the village alive.
There appears to be many months of difficult trading ahead for the bars and restaurants hopefully they will still be there when I return.