I honestly can’t remember seeing a lot of documentary series about the making of games, but this series was fantastic and it really gave me a completely new appreciation for what goes into the making of PC games.

Double Fine Adventure!
https://www.youtube.com/playlist?list=PLIhLvue17Sd7F6pU2ByRRb0igiI-WKk3D - playlist from the official DoubleFineProd YouTube channel (21 videos, 20-30 mins average per episode).
The blurb:
“This landmark documentary series allows everyone to share in the passion, humor, and heartbreak of making “Broken Age”! Featuring twenty episodes plus a host of new special features, the Double Fine Adventure is the most in-depth look at video game development ever produced.”
They also made a similar 3-part making-of for the remastered version of Grim Fandango (one of the most origina PC games ever made IMO), as well as the remastered version of Day of the Tentacle & Full Throttle.

The Making of Grim Fandango Remastered
Part 1 - https://www.youtube.com/watch?v=JNhE7zxJymE (12:30 mins long)
Part 2 - https://www.youtube.com/watch?v=s-LSMCYjHow (7:30 mins long)
Part 3 - https://www.youtube.com/watch?v=y4rOgZ3RINE (15 mins long)
from the official DoubleFineProd YouTube channel.
The blurb:
“The makers of Grim Fandango discuss the development of the beloved adventure game and how Double Fine Productions plans to release the original classic to a contemporary audience.”

The Making of Day of the Tentacle Remastered
https://www.youtube.com/watch?v=LjF4eMrYfG0 - from the official DoubleFineProd YouTube channel (12:30 mins long).
The blurb:
“Originally released by LucasArts in 1993 as a sequel to Ron Gilbert’s ground breaking Maniac Mansion, Day of the Tentacle is a mind-bending, time travel, cartoon puzzle adventure game in which three unlikely friends work together to prevent an evil mutated purple tentacle from taking over the world! Now, over twenty years later, Day of the Tentacle is back in a remastered edition that features all new hand-drawn, high resolution artwork, with remastered audio, music and sound effects”

The Making of Full Throttle
https://www.youtube.com/watch?v=iS9_HODguwU - from the official DoubleFineProd YouTube channel (11 mins long).
The blurb:
“Tim Schafer recounts his time at LucasArts and the many influences that helped create the classic adventure game Full Throttle.”