All Model 3’s are built “for inventory”. Tesla does not build a specific Model 3 configuration just because you placed an order for one. They use the backlog as a general guide for how many of each color and trim combination to build and once a VIN has been assigned they match the VIN to an existing order for that configuration.
Tesla had excess inventory at the end of Q3 (yesterday). In the past they have frequently sold out of every vehicle by quarter end, but production capacity has been increasing so it is not at all unusual for them to have cars in inventory for immediate delivery, just like any traditional dealer would have.
There is no issue with purchasing a car that has been in inventory for a month. The cars are not so fragile that sitting for a month would cause them any harm. Depending on your location you may be able to request to look at the car prior to hitting the purchase button. In my location the cars are stored offsite at a lot that is not accessible to customers, so there is no way to look before you buy. But if you purchase the car and then don’t like the condition once you see it at the delivery center, you can reject it and wait for them to assign you another one.