There is nothing like this in hl1, most of the behavior is hardcoded in the game's SDK and pretty generic. I.e every breakable object has 'health', and the barrels explode when they hit 0, explosion is just an instant calculation of dmg property to distance falloff, and there is very basic "if then" scripting language that you have to configure manually. The engine is not very far from quake1 because technically it is one. Source: a former hl modder.
Underlying implementation is a pure rendering engine, network and some low level core stuff (which is, as I mentioned a facelifted quake1 engine). All game logic including weapons, enemies, ai, bullet and explosion raytracing and game objects called 'entities' are contained in SDK and being compiled to client.dll and server.dll for server side and client side respectively. What derefr is talking about I have no idea because it has not a slightest relation to the game (no offense)
