A key is a dominant tone. It's also equivalent to a singing do in a key. The key C you mentioned is do, which is a fixed-tone singing method. That is to say, sing 1234567 regardless of the key.

but the first key is different. That sound can be used as do. What tone is 1 in any mode, push it down. It is because of the first tune that there is a rising and falling mark of the mode. They are all shifted against C.

as in c key, 1 to 2 are full tones. 2 to 3 are full tones, 3 to 4 are semitones, 4 to 5 are full tones, 5 to 6 are full tones, 6 to 7 are full tones, and 7 to 1 are semitones. You can read all, half and all. The other keys are all in this relationship. It is done by raising and lowering marks.

Extended information:

The name of the key is based on the tonic. For example, if the seven natural tones are arranged in order from C, the key is C. If the key is moved up by a pure fifth, it is the key with G as the main tone. Moving c down by a pure fifth is the f key with f as the main tone.

Generally, different tunes are distinguished by the key signature marked after the clef at the beginning of the score, and different modes are the specific forms used in music works.

Generally speaking, any key includes a major with the same name and parallel minor of this major.

Besides, a key can also include some other modes that are not commonly used. For example, the medieval mode in medieval Europe and the national modes in other countries, such as the traditional five-tone mode in China.