Someone already provided a better answer than I can but just as another example. Constants can sometimes be a tricky name because in programming we think of constants as having a set value, but that’s not entirely what physicists mean. They mean values that have no units. Pure numbers as it were. Sure these may change or we may measure them better but by redefining our units in terms of these constants we kinda sidestep that. There isn’t anything to measure if that makes sense. The only thing to measure is what a meter is.