"Where do you store it? How do you organize it there? How do you keep track of your inventory?"

I am sorry but these questions come across as extremely silly - this is not rocket science, even manufacturers of beer have to figure out these things, if you can't manage a warehouse, you would not be able to be any kind of serious manufacturer in the first place.

They're not difficult, but they are expensive. Which is what ends up in prices.

Check warehouse rates or spot freight shipping.

It is not as silly as it sounds. I am working for a big manufacturer (of products that don't require any spare parts) and the warehouse space is minimal, sometimes raw materials from the suppliers are unpacked and weighted directly on the production line, not going through any warehouse; the finished products are wrapped at the end of the line and loaded directly in the trucks, not going in a local warehouse.

That means if a new requirement appears to add something in the warehouse, space needs to be added. In some cases space does not exist around the plant, so you need to get some space in a warehouse somewhere else, most probably managed by a third party with higher costs, then integrate an inventory management software with that third party warehouse. This is already very complex for a manufacturing plant with zero budget for IT projects and the cheapest IT people needed to keep the lights on.

Wow, no one says you have to get a spare part to a customer tomorrow morning. Just grab one before it goes to assembly line, ship it with the actual completed machines to a distribution center where it will go to the customer from there. There's nothing "new," except at the beginning/end of the chain, just add a few more of those parts to the next order of them. If it will impact operations to take a few from the beginning, some of those spares will have to be delayed. That's ok, tell the customer 6-8 weeks to deliver a spare part. That was pretty standard shipping times when I was younger.

> Just grab one before it goes to assembly line, ship it with the actual completed machines to a distribution center

So customers can only order replacement parts while assembly lines are currently doing production runs?

This is sort of liking saying “how hard could it really be to do a special software build for one customer?”

I guess it depends on how you CI and build process is setup. Caddy does an excellent job of doing per-customer custom builds.

If you just "grab one before it goes to assembly line" this will likely have a huge impact on the assembly line. Factories put a lot of effort into managing inventory of material. Lead times on parts can often be in the months if not years. If there are unexpected shortages of parts and subassemblies, the hourly or daily costs of a line down can be enormous. Shipping one of something to an end consumer is usually a completely different supply chain than shipping containers full of finished goods to distribution centers which then go to retail. Taxes, tariffs, shipping methods, etc are all completely different. You can't just commingle things like this easily.

We once had to drive special parts for one car by taxi cab to the factory, in single figure quantities, to not let the line stop and pay the fine... "just in time" production can be hell...

What if it's no longer being manufactured or on assembly line? HOw do you grab it before assembly line?

And comingling parts with completed machines? Heh. You have no idea of the distribution chain of b2b retail goods vs b2c service repair and such.

6-8 weeks repair time is still common, especially nowadays.