Design finalized means all the different modules have been locked, manufacturers for those modules identified, vendors selected and contracts signed to buy/build everything that will go into the block-3.
This means everything from the fuselage, wiring, electronics down to the bolts that hold the seat.
Now all that is left is to place the order as such and start building it.
This may seem like a non issue to laymen, but for those familiar with project management and supply chain, it is a big achievement.