Do you know what is this type of graphic design called? More precisely I mean characters with "shadows", any Youtube tutorials? I use Adobe Illustrator. Here are some examples:
That Poseidon emblem could be understood as a simple 1-color or 2-color linocut or woodcut style, mainly because of the sawtooth shading technique. The linked examples are less traditional and might simply be referred to as minimal flat 1-color or 2-color spot illustration. To locate some tutorials search the following phrase: "1 color linocut illustration vector tutorials" and view the resulting images. The results should be in the same visual neighborhood as your examples and hopefully some are associated with a useful tutorial. Swap out "woodcut" for "linocut" and search again. Also, searching for 'linocut' or 'woodcut' vector brushes may be beneficial. Good luck.