Adobe Premiere Pro 3rd party plugins , royalty free videos, and special effects

Yes, Premiere clearly runs on multiple cores ... best current data from Puget Systems & other places suggests the max usable cores to be somewhere between 12 and 16.

Yes, it has been able to use multiple GPUs for some time but is not particularly good if they're connected in SLI mode.

I'm not sure what you mean by "Adobe supported" ... but there are a ton of plugins for Premiere, it's one of Premiere's great strengths actually. I've the Red Giant suite and Neat video noise reduction plugins, Autokroma for BRAW, and a couple others. Borisfx.com has a number of things inlcuding their Sapphire line.

Neil

See here:

https://www.prodad.com/Home-29756,l-us.html

and here:

https://www.videoblocks.com/

and here:

https://www.pond5.com/

Special effects are best done in After Effects.