As far as I'm aware this is how grid operations are done in the overwhelming majority of the world. You spin a generator up to speed, match phases and close the switch to connect it to the rest of the network. Due to how electricity works, if any connected generator is slightly out of phase with the rest that will generate currents causing it to pulled back into phase in a closed feedback loop. So while it is technically possible for a generator to be out of phase with the network, it can never be outside of phase for very long because literally all the other generators on the network are pulling it back into phase.

If the grids are particularly far from each other and/or with differing frequencies, a DC high voltage line can be used instead but that is certainly not the most used method.