Most of them are homemade but with a server like this they might give the list but not the programming.
Its a mix of both if a user suggested a plugin that the admins and mods like they'd will implement it and if a user suggested a idea (e.g. flying in town) and the mods and admins like it the may program a custom plugin for the server. Ps I know the fly plugin won't happen it was the only thing that poped into my head.