Or do I need to open up the externals to see what types / speed drives I have in there? Also, if I can just buy the enclosure, the drives are just plug and play in there right? I don't need to download drivers or format them or anything right?
4TB 2.5" drives and a big WD 8tb 3.5" drive that are old USB 2.0 externals in their factory enclosures.
I don't think any drive manufacturer sold 4TB or 8TB drives in USB 2.
Did you mean USB 3?
or do I need to buy an actual USB C drive like this?
USB-C is a connector type. It actually allows for USB 2 speeds. (30MB/s)
If you're going to buy anything get simple USB 3.0 adapters that support UASP. You'll get more than enough performance out of those.