Using a 27" Apple Screen with a 14" MacBook Pro

WHICH 27 Apple screen makes a BIG difference:

if yours has display input and MagSafe in its lead-in cable, it is a Thunderbolt Display, and requires the Apple Thunderbolt-3 <-> ThunderBolt-2 adapter, which does not work with any other display protocols, no matter how many additional adapters you try to add.

If yours is the Mini DisplayPport display, its lead in cord has display input, MagSafe, AND USB-A input, and it requires a Mini DisplayPort adapter.

Here is one:

Monoprice Select Series USB-C to Mini DisplayPort Adapter - Product # 24273 - $14.99

You can try and grab a third party Mini DisplayPort to usb c cable.

I have it working with such a connector, bought from a local parts shop

Apple will generally only produce adapters "when Apple has something special to add". 'Commodity' adapters are left to third-parties.