Is somehow locked physically during production, or through software?
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ROM can be hardwired when it’s made, or it can be programmable (PROM).
The simplest form of PROM to think about is a bunch of tiny fuses that can be accessed by an addressing system, if you put a normal current though, they pass it normally, but if you push too much through, the fuses blow and now when you try to put the normal current though, it doesn’t work. This lets you program the ROM after it has been made.
The circuit the PROM is connected to needs to be able to generate that higher current to blow the fuses. You could use such a circuit to blow more of the fuses if you wanted but in this basic kind of PROM you can’t unblow the fuses though.
EPROMs are erasable proms that have something more like a circuit breaker instead of a fuse. Different EPROMs have different ways of resetting the breakers. Some have a little window you can shine light into, others accept an electrical signal to reset their breakers and these are called “electronically erasable programmable read only memory” or “EEPROM”
Some EEPROM is arranged in blocks that can be erased separately from one another. This is what ‘Flash Memory’ is. With some additional cleverness wrapped around it to read the block before erasing then writing it back with modifications, you can treat it as writeable storage like a hard drive.
In the case of cartridges, the device reading the cartridge probably can’t generate the current needed to blow more cartridges, or direct specific patterns of into it. The ROM inside it was programmed with special hardware either before being soldered into the cartridge or after.
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