There's misleading stuff on the internet about the options with injection moulding tools (terminology: the moulds are called tools) and this article doesn't help clear that up much.

For a normal hard plastic like ABS, you basically have two options: a "soft" tool or a "hard" tool. These are both usually made of steel, even aluminium can be too soft at temperature and pressure for good parts.

The difference is that a soft tool is a fairly normal steel that can be milled on a standard 3D CNC machine. This makes it maybe $10k and a few weeks to make, given a suitable design. You can make probably 1000 parts with a tool like this (maybe more depending on the part and the tolerance needs).

A hard tool is made from a very hard "tool steel". There are some different alloys in use but there all extremely difficult to machine - the main method f is Electro-discharge machining. This is a rather involved process, usually involving milling heads that can then spark erode metal to he same shape (look up EDM if you haven't seen it, it's cool). This whole process is more like $300k and takes 3-6 months for a 1-2 action tool. It's likely to be the biggest capex line item for setting up mass manufacture.

For prototyping and low volume manufacture, I strongly recommend Model Solutions in South Korea (https://www.model-solution.com/en/m/main). For us they used to turn around injection moulded parts in quantities of 1k+ in 2 weeks (including postage), at very reasonable prices (used to because I haven't needed them for a couple of years).