Perhaps this article will help you understand.
On the western continent, countries use E, F, J, or L grounding plugs and receptacles. Despite differences in the recess geometry and the grounding means, all are similar with respect to the conducting pin geometries.
Type E, used in France, Belgium, Poland, and Czechia, and Type F, used in the rest of western continental Europe except Switzerland and Italy, have two 4.8 mm (~3/16 in.) diameter pins 19 mm (~¾ in.) apart, center-to-center.
The Types E & F plugs are variations of a style call "Schuko". In practice today, most Schuko plugs incorporate the grounding means for both the E and F types and are used interchangeably in most countries.
Type J, used in Switzerland, and Type L, used in Italy, have 4.0 mm (~5/32 in.) pins on the same centers. Except for the recess and grounding means, the only difference between all of these plugs is the diameter of the pins. 4.8 mm pins will not fit in the pin holes in Swiss and Italian receptacles; 4.0 mm pins will still fit in Schuko receptacles. These receptacles hold the smaller pins quite tightly.
The type C, "Europlug", is ungrounded, has 4.0 mm pins, and fits in the receptacles of every country in western continental Europe. The adapters linked upthread (I like that term) are supposed to be Type C (smaller pins), so they should "fit" all over western continental Europe, but fitting does not mean that they are necessarily safe for all applications.
US appliances with three prong, grounding plugs have the ground pin for a safety reason, and should not be used with ungrounded 2 pin adapters, even in the case of the Zoppen adapter where it accepts a three prong plug. That is equivalent to cutting off the grounding pin on a US appliance in order to use it with a two blade, non-grounding extension cord.
Also, the Type C plug is rated by European codes for a maximum of 2½ amps (575 Watts) and should not be used with hair dryers, which draw a lot more amps.