Coffee bitterness, called Astringency, is usually caused by tannins. We know tannins, for example, from some red wines, where they make you feel as if it is tightening your mouth. But it is not desirable in coffee. The coffee seems to be dry. It can even scratch your tongue like emery.
Tannins and thus bitterness get into the coffee in increased amounts with improper coffee preparation. This applies to preparations where water flows through the coffee, e.g. V60 filters, Chemex, but also espresso. This is not the case when making coffee, when the coffee is immersed in water (eg french press, siphon).
The problem is channeling
When water flows through an improperly prepared puck in an espresso, or through a mass of coffee in a filter, a break can form and the water will start to flow through this break much faster than through the surroundings. This increased flow will cause greater extraction of tannins and thus higher bitterness in the coffee.
The solution is a better technique of preparing the puck for espresso (I will explain in detail in one of the other articles), or, for example, choose a coarser grinding - well usable in preparations for the filter. Coarse grinding has a much lower tendency to form channels.
The second possible culprit is quakers
Quakers are immature beans that you can't tell distinguish in green coffee because they look like others. But when coffee is roasted, they are much lighter than other beans. Significantly lighter. And it is these quakers that can cause bitterness in coffee. Before you grind your coffee, it's a good idea to check it and throw away any quakers.