'Sector' reallocation

I was wondering about this, does anyone know how this works perhaps ..

Bad sector on HDD can be swapped out for single sector, right?

On an SSD in case of bad 'sector' that triggers reallocation, what exactly gets reallocated? Smallest unit we can write to is a page (multiple sectors). But if it also affects ability to erase, we'd need to swap put entire block as it's the smallest unit we can erase. Does this mean we need to reallocate an entire block?