Where can I download more editing tools for iMovie on Mac Studio?

You can't . . . everything available is already in iMovie.

If you need a more professional app you can buy Final Cut Pro for £300 although there is a FREE 90 day trial to see if you like it.

Final Cut Pro for Mac - Free Trial - Apple (UK)

Alternatively you can download the FREE version of DaVinci Resolve which is every bit as good but a bit more complicated to learn as it is a true professional app.

YouTube has countless beginner tutorials for both apps.

https://www.blackmagicdesign.com/products/davinciresolve

What tools are you looking for?

iMovie is a quite good editor but not all its capabilities are readily apparent. Folks here could explain how to do certain things if you identify what you're interested in doing.