Inside the iMac is a 1tb hdd.
What iMac do you have?
Should I use a USB c 3.1 or 3.2?
You have to be careful when referencing and shopping for USB devices.
For some silly reason, the power at be rename the colloquial name for USB3 specifications. So, the names change, but the specification stays the same.
USB3.0 is the same as USB3.1 gen1 and now known as USB3.2 Gen 1. It has max of 5Gbps. Many people also refer to this as USB3.
USB3.1 is the same as USB 3.1 gen2 and now known as USB3.2 gen2. It has up to 10Gbps.
USB 3.2, which is now known as 3.2 Gen 2x2. It has a max of 20Gbps.
Just looking above, anyone can understand why talking about USB can be confusing.
I’m checking on external ssd’s.
There are a bunch of external options and not just USB, all have pros and cons.
USB3 (or USB3.2 Gen1) SATA SSD, is the cheapest option for external, and will give a decent speed increase over your HDD. It would feel like a different computer, and will be much more responsive. You would see real world speeds of about 400MBps. The biggest downside to USB3 (and all USB) is that there is no TRIM support for USB on the MacOS. You can get a 1TB SSD and enclosure or SATA/USB adapter cable for less than $100 in the US.
Depending on the year of your Mac, you might be able to take advantage of the newer USB standards, meaning faster speed, but a little higher cost. Again, there is no TRIM support for USB on the MacOS, so this is the biggest downside.
Another option would be a SSD over TB1, 2, or 3. TB drives support TRIM, so this is a huge plus, but it comes at a higher cost.
They are not as easy to find, especially for cheap, but you could get a TB1 or TB2 enclosure. I have purchased a few TB1 HDD drives, and replaced the HDD with a SATA SSD. The LaCie Rugged TB drive is a good example, you can replace the HDD in it with a SSD in less than 2 minutes. If it is TB1, you will see slightly faster real world speed than USB3, but having TRIM support is the main sell. I never tested the TB2 model, so I cannot comment on the speeds.
The fastest external drive possible without getting into striping RAIDs would be a NVMe SSD over TB3. Depending on your Mac and if it has a TB3 port, you could see speeds almost 3000MBps using a TB3 NVMe SSD. The biggest downside to this is cost. The enclosures start about $150, and that is without the NVMe drive.
You can use TB3 NVMe drives on older Macs too. I am currently using a Samsung X5 NVMe TB3 drive on my Late 2012 iMac. It is the fastest possible drive on my iMac, internal or external. I get almost 900Mbps max speeds.
If you Mac does not have TB3 port, I wouldn't recommend getting a TB3 drive unless you are looking for max speeds, as there is more investment due to the TB1 and TB2 ports not providing power over the bus to TB3 devices. Basically, you have to get a TB3 dock in addition to the TB3 drive to power the drive.
Let us know what Mac you have, and that could help narrow down the pros and cons of your options.