This title may sound a bit strange, but the question behind it is very serious and has caused numerous debates between UI designers.
When a folder or a menu group is collapsed should the arrow point to the right, or downwards?
This is an example of the arrow point to the right when collapsed and downwards when expanded. However not everyone believes in this standard, so UI designers fight that the arrow should be down when collapsed and up when expanded. As you can see in the article image above, the 2 options are both valid, however option 1 (on the left) has dominated the technology space as Microsoft (with Windows), Apple (in macOS) and pretty much all the Linux OSs all use this notation for folders, groups of contacts, and any other kind of grouped lists.
The web however is a different beast and a lot of designers come from print, so they know how things should work in the real world, but no so much how the standards have been set in computers for the past few decades.