Question about WD Blue SN570

No. A standard M.2 SSD will not work in an iMac without the use of a third party adapter, as they didn't have M.2 slots back in 2014. Plus you need to make sure the iMac's firmware is up to date so it can communicate with an NVMe SSD. You will only be able to use macOS 10.13+ on this iMac when used with an NVMe SSD.

You would be better off with a standard SATA SSD, something like WD Blue (1000 GB, 2.5") as it will get in place of iMac's HDD. You can find how-to instructions on YouTube.

I use this on my MacBook Air (early 2015) with an adapter (cause Mac doesn't have a standard M.2 interface). It works fine so at least it is compatible with Mac.

As I know, *some* 2014 iMac support Fusion Drive, i.e. there are SSDs in these iMac. (Theoretically) you can buy an adapter (M.2 to Apple SSD socket) and install this WD SSD to your iMac, but actually I don't know wether it will work. Fusion drive is a system of both SSD and HDD and I'm not sure if it can work after you changed on of it. And as @boriskhv has pointed out, make sure your mac is up to date because it is said that earlier Mac OS may not support NVME specification.

I also don't know whether there is an SSD socket on iMac of model doesn't have Fusion Drive.