iCloud & iCloud Photos · Online-only files and Optimize Mac Storage
How do I keep an iCloud Drive folder always downloaded on my Mac?
Short answer: Right-click the folder in Finder and choose Keep Downloaded (on recent macOS). In Storage Studio, the automatic cloud step is opt-in per provider, so you can leave iCloud Drive switched off in Preferences → Rules.
In recent versions of macOS, right-click the folder in Finder and choose Keep Downloaded. macOS then keeps that folder's files on your Mac even when it's freeing iCloud space.
If you use Storage Studio's optional Optimise cloud storage locally step (in Preferences → Rules → Free Up Space includes), it's switched on per provider, so you can leave iCloud Drive off there if you rely on local copies.
Bear in mind that Run Tuhi also has a step that frees large downloaded cloud files. If there's a folder you always want local, mark it Keep Downloaded in Finder so macOS keeps it on your Mac.