To my knowledge, the HL1:EP1 game files should be located in Half-Life 2 folder if you go to Steam > SteamApps > Common, but the only two folders I see relating to episodic content is "ep2" and "episodic" neither of which contains many of the types of files/folders that regular Half-Life 2 has. Not as far as I know anyway.
Half-Life 2: Episode One is the first in a new trilogy of episodes that are scheduled to be released over the course of the next year. the game does include a developer commentary system of

While sadly not an official Valve product, it is in fact the next version of the excellent Half-Life 2: Update mod from the same team – which will presumably include all the Episode DLC too.

Users discuss the inclusion and value of Half-Life 2 episodes 1 and 2, and the difference between them and the main game. Find out the release order, the story, the price, and the mod compatibility of the episodes.
Valve's games made prior to Left 4 Dead all include the Half-Life 2 Shared Files. One of my old answers breaks down how the GCF files were laid out for TF2 specifically. There are 2,722.83MB taken up by shared files, including the multiplayer ob binaries.gcf file, which would include the version of hl2.exe used by all of Valve's older
They can just end Gordon's story and the whole phase of half life being human resistance in earth against the combine, kinda like half life 1 went from black mesa to the resistance story. Or they could stop the combine and leave the story in a world where humanity is still fighting the combine in the ruins of earth. EmUe6H.
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    • does half life 2 include both episodes