Detectable vs. functioning are two different things. For cannabis, it is metabolites that are detectable for some time afterwards. They are no longer actively having an effect on the person. These metabolites get deposited into some tissue, like hair or fat, and can be detected later.

Imagine you eat cherries and accidentally stain your hair. This means that the cherry substance will be detectable in your hair for a long time, but any effects of you eating the cherries will be long gone.

Detectable by who? Certainly not by anyone without a lab after a day or so.

And?

And? It's ok if you do not care. But I wanted to make clear that for the body there is a difference between alcohol and other drugs as alcohol, to the body, is mostly just another nutrient (I mean almost all ripe fruits contain a certain amount of alcohol, yet we eat them without problems).

You have that entirely backwards. Our body has receptors for cannabinoids, and our bodies treats ingested sources just as though they were the endocannabinoids we produce naturally.

https://en.wikipedia.org/wiki/Endocannabinoid_system

Meanwhile we don't know of any specific "alcohol receptors", and it seems to just interfere with other standard processes.

You do know that there are endocannabinoids present in your body, right? And also drug tests for cannabis detect the presence of cannabinoid metabolites, not THC itself. Your opinion does not sound informed.

Weed is very natural. Very old.

Alcohol is responsible for many illnesses. It is NOT 'just another nutrient'.

There is a difference between a few slight trails of alcohol vs. what we drink.

"It's very natural" is utterly irrelevant to the question of whether (or to what degree) it may be harmful. Belladonna salad, anyone? Mushroom & pepperoni pizza made with Amanita phalloides?

I just mentioned it as it was mentioned that alcohol is 'just' a nutrition as it is contained in fruits.

Not a good or very useful argument but in comparison to 'alcohol exists in nature' 'so does weed'.