Why can the 24 solar terms be mentioned in the calendar? Is it not fixed?

It is not fixed but can be calculated!

The 24 solar terms divide a year into 24 equal parts according to the position of the sun on the ecliptic (that is, the earth's orbit). That is to say, the ecliptic is divided into 24 equal parts, each equal part accounts for 15 of the ecliptic. Not by date. Because the time required for the sun to pass through each bisection is almost equal, the Gregorian calendar dates of the 24 solar terms are roughly the same: the first half of the year is around 6 and 2 1, and the second half is around 8 and 23.

And there are two formulas: 621 in the first half of the year and 823 in the second half.