What exactly were "Plugins" on early Beta Minecraft servers and how did they work?

For the past few years, I have been assembling an archive of Minecraft videos that are nostalgic to me. A few of the videos in my archive originate from "Pwegoserver" a moderately popular Minecraft server that operated for an unknown amount of time between early Beta and the mid 2010s.

I never got to play on Pwegoserver. By the time I figured out how multiplayer worked in M.C, Pwegoserver had been shut down only months prior after it was sold to a server network known as "Athion" who proceeded to delete all Pwegoserver assets shortly after the acquisition ( I genuinely cannot understand why the sale took place or what was gained from it if everything was going to be deleted ).

Pwegoserver has had a lasting impact on me. In my first and now long-gone Minecraft world, I built a modified copy of Pwegoserver's main hub and many of my builds throughout the years have been inspired by builds on the server.

Anyway, onto the point of this post.

One of the Pwegoserver videos I have in my archive is of the server owner "Pwego" ( real name; Chris ) giving a tour of his server in early 2011. During the tour, Pwego discussed that the server had a few "Plugins" which enforced certain features.

One of these "Plugins" prevented mobs from spawning in or around the server's main hub and another plugin allowed the main hub to be filled with NPC's which had scripted dialogue.

I haven't played on servers in awhile but as far as I know, the "Plugins" that Pwego spoke of in his old videos don't exist in modern servers ( which I think use Command Blocks to do everything ). Therefore, I'm wondering what exactly "plugins" for early Beta servers were, where they came from and how they worked.

I'm not looking to acquire any of these plugins, I just want to know how they worked.