OpenTTD ships with the following: core game files, OpenGFX (graphics), OpenSFX (sound effects), and OpenMSX (in-game music). All other content (AIs, game maps, scenarios, NewGRFs, game scripts) can be automatically downloaded through the in-game content download service. Almost all current mods are now released by contributors through this service, you no longer need to go to the forums or other websites and download individual files.

If you've not played OpenTTD in the last 3-5 years, then you may be pleasantly surprised by all the new features that have been added. Up to 64 types of rail lines, roads and tram lines; up to 128 types of industries that can accept/receive 16 types of cargoes each; support for 32bpp graphics and extra zoom level graphics; improved interfaces, improved game mechanics, better vehicles management. Contributors are continually finding new ways to exploit and expand features. And since OpenTTD's code base moved to GitHub last year, significantly more people have been able to provide patches, suggestions and bug fixes that have been adopted and implemented. Part of that change is why OpenTTD is now available on GOG and Steam, and anytime the core game is updated, those platforms automatically get those updates within hours, if not minutes.

OpenTTD does not come with AI by default, but there are AI's on the BaNaNaS download system.

SimpleAI would be the nearest to the OG (sometimes quite silly) AI.

If you need to download something you can do it under the mod dowload system bundled into the game. Yes, probably you will ned to download some AI if interested in competittive single player games with other companies. Each player with a different AI is possible.

As it's been said the bundled BaNaNaS download system is very powerful, it forms part of the proper game/Interface and there you can download not only AI's, but scenarios, maps, heightmaps, skins, new vehicles collections, graphics, music, even the HD graphics packs. All you need into the game interface.

Greetings