They may fit physically but they are often wired differently, which you will not be able to see since the cables are often wrapped or braided.

The power plugs that go into your mainboard, SATA or Molex devices, fans and so on are standardized. But, while power supply manufacturers likely get the actual plugs that go into the PSU from the same supplier, there is no standard for their wiring, so their pins will not always correspond to the same pin on the standardized plug.

You MAY be able to mix and match between different power supplies of the same brand. Do your homework! Ask the manufacturer's customer service, perhaps check if the company offers manuals that allow you to compare the wiring, or get a multimeter and check if they're using the same patterns yourself. Be very careful that you're getting it right, because just a fraction of a second is enough to destroy something if it's getting too much power or the voltage is too high. If you turn your rig on and then notice something's wrong, it's already too late.

I'm posting this because a friend of mine upgraded his PSU last week and, being a bit lazy, he just exchanged the two units since the cables of his old PSU happened to exactly fit in all the right slots on the new one, even though they were not even the same brand. He turned the PC on, immediately noticed a burning smell and turned it off again - but both of his SSDs were already fried. Everything else still works, luckily, and he has now replaced the old cables with the new ones, but there was damage and the replacement (just one 512GB SSD) will cost him 70€. Had he been less lucky, his entire PC might have been fried.