I’m making a city and as I add streets and lights and buildings my unity gets slow, and I did not make a quarter of what I have thought. How do companies do cities like those in GTA and don’t get the engine slow during development?
Because GTA was built my a team of hundreds of very experienced developers with a budget of millions? There’s nothing to stop you doing the same in Unity, but I get the impression from your question that you might be quite new to game development, so I’d set your sights at something more realistic to start with.
Let’s start with the basics: are you GPU-bound or CPU-bound? What are your profiler stats? Have you followed all the steps at Unity - Manual: Graphics performance fundamentals ?