I used to work at Boeing's space systems division.

Studies I worked on showed 98-99% of space projects could be built from materials already in space - from the Moon and nearby asteroids. The other 1-2% are either too rare to mine in space or too hard to make up there. It is just easier and cheaper to launch from Earth. This would vastly reduce how much you need to send from Earth, and lower the costs dramatically.

Simple products like water and bulk regolith would be first. Water as water and oxygen are needed for life support, and makes up a high percentage of rocket fuel. Bulk rock can be used for radiation, thermal, and impact shielding. Impacts come from micrometeorites and stuff kicked up by rocket exhaust.

Over time you can add other products, including making parts for your space factories. Then the factories themselves can grow without shipping them all from Earth.