SAMSUNG 870 EVO SATA III SSD compatible with imac 27 late 2014 ?

It is not quit that simple or easy.

You need a kit with a temp. sensor cable, a sled and new display tape at minimum.

see > https://eshop.macsales.com/item/OWC/K27IM12HP1TB/

Another option, instead of opening and risking damage.

Is to get an External USB 3.0 SSD to startup and run the 27" 2014 iMac on that.

see > Use an external SSD as your startup disk … - Apple Community

I use some as external and they work great. Not sure about internal.