There are two types of ways for those people to make up for the days of fasting that they missed. They can either fast those days later after Ramadan, or they pay for one poor person for every day they didn’t fast, or both.
For short-term sickness, travel, and a girl’s monthly business, the person must fast the days they missed afterwards.
Long term sickness must do the payment
People with chronic illness don’t have to do so
On another note, postpartum women have to do both