CMYK refers to Cyan, Magenta, Yellow, and Black. These four colors are the primary colors and used in printing the color image. These color combinations are used in the press printing process and are also called full-color printing or four-color printing. You can produce over 16,000 different color combinations.
The C, M, Y, K are the percent values of Cyan, Magenta, Yellow, and Black, ranging from 0 to 100 percent.
The printing machine draws the image by combining them to various degrees to get the desired result.
Therefore, the question is why "K" represents the black in CMYK.
Technically speaking, adding an equal amount of Cyan, Magenta, and Yellow should produce the black color. But due to the ink color impurities, the actual black color is difficult to create due to impurities. Therefore, black is the key plate that brings the details in the image. It properly aligns with the other three colors. Thus, in CMYK, black is the key color and is represented by K to avoid confusion with blue in RGB.
CMYK is a subtractive color system, which means the more you add, the more you get closer to black.
That is true that the CMYK color system is used in the printing process. Use it at the small scale level where the color accuracy is not that significant, as most of your inkjet printers in house or offices.

In HEX, the HEX color code in the Hexadecimal representation of the color starts with the hashtag (#) followed by the six letters. The letters can be numbers, alphabets, or a combination of both. The HEX color code shows the combination of Red, Green, and Blue. In HEX, among the six colors, the first two colors represent red, the next two letters represent green, and the last two letters represent blue.
HEX is an additive color system, which means the more you add, the more you get closer to white.
That color system is safe for web browsers because what browser you use will provide you with the same results. It commonly uses in web designing. These Hexadecimal color codes are HTML color codes.

It is essential to know the difference between different color modes because they help plan and optimize your design process stage.
CMYK is related to the printing press, but at a small scale, where the color accuracy is not a significant problem. That color model is used where you need physical printing. In comparison, HEX is used for web design purposes. HEX color codes are primarily used by web designers to design the website, logos, banners, ads, etc. It is a short form of RGB color.