The most fundamental difference between regular coffee and Americano is their preparation method.
While both coffees are black, the Americano consists of an espresso shot and hot water, while the regular coffee is simply hot water filtered through coffee grounds.
And while you need an espresso machine to prepare an Americano, regular coffee can be made in a variety of brewing methods, some of which include drip machine, French Press, Moka pot, Chemex, V60, or others.
The brewing methods also affect the taste of the final beverage. The Americano has an intensely deep flavor, full of nutty and chocolatey tasting notes coming from the espresso shot.
On the other hand, the taste of regular coffee depends on the particular brewing method.
While the coffee made in a French Press will taste robust and bold, the pour-over coffee will have a much cleaner taste. So there is more room to play around with flavor with regular coffee.
Another difference between them is the amount of caffeine. The caffeine content of an Americano will depend on the number of espresso shots used, around 60-120mg.
A cup of regular brewed coffee contains 95mg of caffeine on average. But since people tend to brew more than a standard cup of regular coffee, a large cup can contain up to 150mg of caffeine.
Hope that helps!