There are multiple types of USB-C cables and multiple kinds of HDMI cables which do not all work for your data transfer needs.
It’s outrageous that most of these cables are not labelled, but that’s where the state of cables stands.
So here’s what you need to know:
If you have a hard drive or high bandwidth device to connect to your computer (like a new camera) for fast data transfer, you’ll need a Thunderbolt 3 USB-C cable which is typically (but not always) marked with a lightning bolt ⚡️ and possibly a number ‘3’. You also need a Thunderbolt 3 port to plug this into.
For example, the ports on the left side of Apple’s 2016 and 2017 13" Macbook Pro were Thunderbolt 3 compatible, but not the ports on the right side. Reference: https://www.macrumors.com/2016/10/28/macbook-pro-tb3-reduced-pci-express-bandwidth/
As of 2018, all the latest Macbook Pro models are Thunderbolt 3 compatible on both sides, and the latest Macbook Air ports are all Thunderbolt 3 compatible. All 15" Macbook Pros that shipped with USB-C have been Thunderbolt 3 compatible and the new 16" Macbook Pro continues this.
Any USB-C cable you buy that is not a Thunderbolt cable (usually a sign if it costs less than $20) is likely a charging cable only, which means it will slow down even a standard hard drive you connect to your computer, and radically slow down an SSD drive. For a standard disc drive, you can expect speeds to drop in half, from ~70MB/s to ~35MB/s; while an SSD will go from 500MB/s (or faster, depending on your SSD) to 35MB/s.
You’ll also find that these premium Thunderbolt 3 cables that ship with new hard drives are only about 3 inches long, and if you try to buy a longer one, they’ll max out at under 2 feet long. The short ones that ship with hard drives are simply because the manufacturers are cheap. The under 2 foot length is to maintain high transfer speeds as a passive cable. To support 40Gbps at a longer length, are active (and significantly more expensive).
Another important note: the USB-C cable that ships with a Mac laptop for charging is not a fast data transfer cable.
There’s been a lot of dongle talk with Apple’s switch to USB-C. In fact, with most of your devices (hard drives, printers, MIDI devices) you can just buy a relatively inexpensive new cable for that device with a USB-C end to plug into your computer. It’s only with hard drives or other high speed data connections (computer-to-computer or camera to computer, for example) that you‘ll need the Thunderbolt 3 cable to ensure fast data transfer speeds.
And now a quick look at HDMI
With HDMI, the issue arrises if you’re trying to play 4k content. You will need a higher speed cable (4k) or highest speed (8k). These cables are either 18Gbps or 48Gbps cables. Up until recently, the typical HDMI cable speed was 10.2Gbps and that is not fast enough for less compressed 4k content.
None of these cables appear to be marked, so it’s all about what you order and then keeping track of them from there. A general rule of thumb: if you last purchased an HDMI cable 3+ years ago, you can bet it will not be fast enough for 4k.
So there you have it. All the USB-C and HDMI cable nastiness due to changing standards, poor labelling, and a general lack of detail in the market. Hopefully this helps with your next hard drive or TV purchase!