

There are couple of factors that makes this a confusing topic.
Vim: On a high level, normally Vim (and Neovim) have their own clipboard system. Vim has multiple internal clipboards that can be used like variables and accessed with other commands. So its kind of sandboxed from your system. But you can explicitly use the commands to access the system clipboard. There is a configuration you can set to use the system clipboard by default.
Linux: Unlike Windows, in Linux we also have two kind of clipboards: the “system” clipboard as you know and the “primary” clipboard. This has nothing to do with Vim and is a feature on Linux systems itself. If you in example in your browser mark a text without copying, it is automatically copied into the “primary” clipboard. Then you should be able to access and paste it with middle mouse button in example. The system clipboard where you explicitly copy stuff is not affected by it.
You should read following documentation: 09.3 The clipboard - Neovim
Another maybe is they might outsource some work heavy stuff.