Why "USB-C" tells you nothing about video
The USB-C plug is a shape, not a promise. The same oval connector sits on the end of a charger lead, a phone cable and a cable that can drive an external monitor, and from the outside they can be impossible to tell apart. Anker UK puts the reason plainly: "the physical port standard allows for many different features, but manufacturers choose which features to include." One lead in the drawer might do nothing but charge a device and copy files slowly; another carries a full video signal as well. The connector itself gives no clue which one you're holding, so choosing a cable for a monitor is really a question about what the packaging says, not what the plug looks like.
The three points that all have to agree
For a picture to reach the screen, the video path has to be supported in three separate places: the port on the laptop, the cable in between, and the input on the monitor. Miss any one of them and the screen stays dark. Writing about a MacBook, BenQ says "both the MacBook and the monitor must support DP Alt Mode" (DisplayPort Alt Mode being the arrangement that lets a USB-C connector carry a DisplayPort video signal), and the same holds for any laptop. It also cautions that "not all USB-C ports support DP Alt Mode, and many brands don't clearly label whether they do," so the first thing worth checking is the laptop itself, not the cable: a port that charges a phone perfectly well can still be, as BenQ puts it, "data-only."
Reading the cable's own label
Assuming the laptop and monitor can both do it, the cable is the part people get wrong most often, and it comes down to reading the description before you buy. Anker UK's advice is to buy on the markings: a cable described for "Thunderbolt 3 / Thunderbolt 4" or "DisplayPort Alt Mode," or one that states a plain "4K / 8K Video Output," will carry a picture, and it flags phrasing like "supports 4K at 60Hz" as the kind that confirms it. A cable sold as USB4 carries one too, by the standard rather than by any single label. The signs that should make you assume no video are just as clear. Anker UK lists "charge-only" among the "terms that do not guarantee video support," and a listing that mentions only fast charging and a slow data figure is describing a cable that was never built for a display in the first place. Its own explanation for the difference is physical: "to transmit a display signal, a lead needs extra internal copper lines," on top of "specific data standards, such as DisplayPort Alt Mode or Thunderbolt."
When a monitor "won't detect the laptop"
This is where the theory earns its keep, because a very common cause of a USB-C monitor reading "no signal" is also the cheapest to fix: the wrong cable. If the laptop charges from the lead but no picture appears, that on its own tells you power is getting through and video isn't, which points at a charge-only cable rather than a broken port or a faulty screen. The reliable test is to try a cable you know carries video, plugged straight from the laptop to the monitor with no dock, hub or adapter in the way, since each of those is another link in the chain that has to support the video path. Charging proves the power lines work; it says nothing at all about whether the display lines are even there.
Which cable to reach for
If the laptop and the monitor both handle DisplayPort Alt Mode, a cable sold for USB4 or Thunderbolt is the safe default, because both standards carry video by definition and you don't have to decode a features list to trust them. The Thunderbolt 4 cable guide lists leads proven to pass a video signal and not just power, and if you're unsure which standard your ports actually run, the USB4 vs Thunderbolt 4 explainer shows how to read them.
For a permanent desk, where one cable also has to power the laptop and reach several devices at once, a dock is usually the tidier answer than a bare cable; the USB-C docking station guide is chosen on that need. Where a monitor only takes DisplayPort or HDMI at the back, you want a proper video adapter rather than a plain lead, which is what the USB-C to DisplayPort adapter guide is for.
FAQ
Q: Will any USB-C cable connect my laptop to a monitor? A: No. Two USB-C cables can look identical yet do completely different jobs, because the cable also has to be built to carry a video signal. Anker UK's answer to this exact question is "no, they do not," because a maker decides which features go into a given cable. Choose one sold for USB4, Thunderbolt or DisplayPort Alt Mode, and treat a charge-only lead as no video.
Q: How can I tell a cable carries video before I buy it? A: Go by what the listing states it can do. Anker UK says to look for terms like "DisplayPort Alt Mode," "Thunderbolt 3 / Thunderbolt 4" or "4K / 8K Video Output," and phrasing such as "supports 4K at 60Hz." If a listing mentions only fast charging and a data speed, it's describing a cable that won't drive a display.
Q: The cable charges my laptop but there's no picture. What's wrong? A: That pattern usually means a charge-only cable. Power and video run on different lines inside the lead, so charging tells you the power lines work but proves nothing about video. Swap in a cable you know carries video, connected straight from the laptop to the monitor with nothing in between, and the picture usually appears.
Q: Do I need a Thunderbolt cable specifically? A: Not necessarily. A cable sold for USB4 or Thunderbolt carries video either way, so either one will drive a monitor, and a cable that states DisplayPort Alt Mode works too. Thunderbolt earns its price when you also want a guaranteed top speed for fast storage and more than one screen at once.