What sound does the seven-string bass make on each string? If you know something, help me solve it. Thank you.

For 7 strings, because the bass has reached B, you can't add bass. According to the fourth degree of bass, you should add a fourth degree of c and f,

These seven strings are b, a, d, g, c, f,

7-string bass is not easy to buy strings, and it is of little practical use. Six strings are enough.