Examples of type 1 (characters involved in production)
In The Gamers Alliance, the Midnight Circus ends up performing William Veer's play The Merchant of Scundia to an Alentian and Sarquil audience. The majority of the actors are actually diguised demons who plan to use the play as a means to subtly affect the audience with their dark magic and turn them more violent in order to make Alent descend into chaos. Meanwhile the Sarquil merchant Javan al-Kassis who has become the demons' captive and has ended up directing the play because the demon infiltrators don't know how to do so, is trying to subvert the demons' plans in a subtle manner by trying to make a statement about the dangers of prejudice and talk about tolerance between different rances while he's playing the part of the demonic moneylender Cravenbolt.
Echo Chamber, the TV Tropes vlog about tropes. The audience never sees the finished vlog; we only see the "making of" aspect.
Minor character Lakitu is the news anchor for Mushroom Kingdom News, a news show in There Will Be Brawl. This also falls under type 3.
The Comedy Web Series Randomocity is about the making of a tv show that is also called Randomocity.
Examples of type 2 (characters are fans)
Magical Princess Sailor Rose Wand (the exact name varies from use to use) in Sailor Nothing, a thinly-veiled spoof of Sailor Moon that apparently crams even moreMagical Girl tropes than the original (which, apparently, doesn't exist in the Sailor Nothingverse). Used to constantly compare "reality" to TV.
The Reading Room has the hugely popular soap opera The Bird and the Birdiful, an over the top parody of The Bold and the Beautiful.
Two of Brad Jones' other shows, The Big Box and Kung Tai Ted, are watched in-universe by The Cinema Snob as regular TV shows.
Examples of type 3 (SWAS is plot point)
The Mushroom Kingdom News occasionally seen on There Will Be Brawl serves for dropping plot points to Luigi.
The Legend of Hayaoh in Tasakeru. Overlaps with Type 4.
The play Thalia sponsors at the Pythian Games in Thalia's Musings. It provides a safe means of revealing Eros and Psyche's whereabouts, repenting to Aphrodite, and asking that Psyche be made immortal. Aphrodite and Hera take to the stage to grant both requests.
The characters of Marble Hornets were originally creating a movie called Marble Hornets. Finding out what happened while the movie was being shot is an important part of the plot. Overlaps with type 1.