Each has a 'Danger Rating', this being the likelihood (or lack thereof) of viruses being disguised as mods on these websites. This is a list of websites where mods are typically posted, and where it is recommended to download mods. Different mods change different parts of the game, so some mods can conflict with other mods and result in a crash, bizarre functionality, or glitches.
It is also possible to corrupt save files or make the game crash. Below is a list of sources where it is recommended to find Civilization VI mods. Somebody may claim that a website has a mod, but instead will install a virus on your computer. DO NOT download files on the internet without certainty of what it is and what it does. Modding, if you do not know what you are doing, can be dangerous.