You need to get something like this. I have no particular recommendation for or against that specific hub, it is just an example. USB-C is a mess, because it is three ports in one. It is Thunderbolt (display and fast stuff), USB, and power. Any particular USB-C hub might give you one, two, or all of those. When the hub says it has power delivery, that means that there is a USB-C port on the hub, which will pass power through to your Mac’s USB-C port. So, using that hub as an example, you can plug in your monitor with HDMI, connect an external hard disk to the hub, plug the power into the hub, plug the hub into the Mac, and still have one port on the Mac free. That is convenient, because to move the Mac, you just unplug one thing, and can go (on battery power). You could also plug a mouse and keyboard into the hub.
Things get a little more confusing, though. If you already have an external drive you want to use, you need to make sure it is compatible with the hub you get. If the external drive is USB2 or USB3 (often the blue connectors), then you will be fine, and can just plug it into this (or another USB-C to USB3) hub. If your external drive is NOT usb, then you will have to find a hub with the appropriate adapter built in, and it is likely to be much more expensive.
If you are shopping for a new external drive, then you should look for one that is either USB-C or USB3. USB-C is sometimes called USB 3.1 or USB 3 Gen2 (or is it 3).
You can plug in a USB-C external drive to a USB3 hub if you have a USB-C to USB3 cable. Cables are another thing which are confusing. There are USB-C to USB cables which are only capable of USB2 and others which can do USB3, because USB3 has more wires connected within the cable. For example, I use a USB-C USB2 class cable to charge my phone in the car, because it was a bit cheaper and is thinner, but I use a USB-C USB3 class cable to connect an external drive to a USB3 port.
This is the setup I created for my wife. She does not use an external monitor, but if she did, all that would have changed is my choice of hubs. She has the Mac laptop with two USB-C ports. We bought a USB-C hub with 4 USB3 ports. She uses the hub to connect her iPad and iPhone, so she just reuses all of the old lightning cables we already had. She has a USB-C external drive, which she generally connects over a USB-C to USB-C cable directly to the Mac, but for the odd times when she wants the external drive and iPhone plugged into the Mac (because her hub doesn’t do power delivery), she has a USB-C USB3 cable to connect the external drive to the hub.
Sorry if I’m not making this simple, but it is a bit confusing. Mostly if you get a reputable hub with all of the ports on it you need, you’ll be fine. Connect the monitor with HDMI, don’t use VGA.