How can normal users use plugins?

We have some plugins installed on our Minecraft server, such as keepinventory. But the plugins only work for those who have surgery. Which plugin you need now, so you can create groups or something that at least have the rights to use the plugins.

For that you need a Permissions-Plugin like PermissionsEx or LuckPerms.

And how does it work with PermissionsEx, it would be nice if you could explain that to me shortly

Just take a look at a few videos on Youtube, because that is very much better explained than I could this xD