help me to know this

I joined the server and really liked what they have at the spawn (AKA The Hub). I want to create something similar, but I’m not sure how they made it or what tools they used. If someone could kindly explain or point me in the right direction with a video or suggest what I should search for, I’d really appreciate it.
I know they’re using a custom resource pack, but it seems like there’s more to it. For example, when I turn off the resource pack, I see some weird stuff, like custom skins on players that turn into big signs.