How do you expand an existing patio while retaining look, structure & level?
Use a level, and compact the base to the correct level so that when bricks are placed on it the resulting surface will be level. Choice of levels ranges from surveyor's total station on down to water in a clear hose. The aim is the same in all cases. Note that the patio may not really be "level" - it should have a very slight slope away from the house to aid in directing surface runoff, as should the new section.

Your title but not the body of your question mentioned retaining the look - that may be the hardest part - finding brick to match old brick is a challenge, particularly if your home is relatively old. You may be able to find a construction materials salvage yard with old brick.

As for maintaining the structure, dig down and see how the old one is built, and match that.