When receiving benefits before FRA, you are supposed to inform the Social Security Administration of the amount of income that you expect to receive during the year and any changes to that expectation so that they can adjust your benefit accordingly.
For the reduction due to continuing to work, note that they withhold entire monthly payments rather than reduce each monthly payment. When you reach full retirement age, the SSA recalculates your benefit by treating you as having begun receiving benefits later by the number of months that benefits were withheld. For example, if you begin receiving benefits at ag 62 and before FRA they withhold 6 months of benefits due to earning income over the limit, at FRA your benefit will be recalculated as if you had begun receiving benefits at age 62 and 6 months.
The SSA has a pamphlet that discusses all of this: https://www.ssa.gov/pubs/EN-05-10069.pdf
