PHB Chapter 7 says this about hit points under the "Constitution" subheading (p. 177):

If your Constitution modifier changes, your hit point maximum changes as well, as though you had the new modifier from 1st level. For example, [...] if you are 7th level and some effect lowers your Constitution score so as to reduce your Constitution modifier by 1, your hit point maximum is reduced by 7.

When your Con modifier somehow decreases by X, your Max Hit Points are decreased by an amount equal to X*Level.

What happens to HP when Constitution decreases? / Are the HP lost when a character is no longer attuned to the Amulet ?

If your Con modifier decreases, your Max Hit Points decrease; any current HP over your new Max HP are lost, too.

No.

The way damage works is that you take it until you get to 0 HP. On the strike that takes you to 0, you check the remaining damage against your max HP, if it's higher or equal to, you are dead.

If you suffer damage while at 0 HP, three things happen. First you suffer a death save failure. Second, if it's a crit, you suffer a second death saving throw failure (three failures means death). Finally if the damage you take meets or exceeds your max HP, you die.

However, the damage at 0 HP only counts for the single hit you are taking. Thus if you take 10 damage, and the next turn take 15, you are still at 0 HP, and just have two death save failures, but no further consequences.