Tales of The Black Freighter in Watchmen. Comic in a comic.

• Nation's Pride, in Inglourious Basterds. "Who wants to send a message to Germany?!" ~Fredrick Zoller

• "I'd buy that for a dollar." in RoboCop.

Station Eleven from… Station Eleven is probably there only one that’s been as interesting to me as the story itself, I guess because it’s such a big part of the story and character motivations. The book and show are both good in their own ways, but I particularly like the passage repeated throughout the show:

I remember damage, then escape

Then adrift in a stranger’s galaxy for a long time

But I’m safe now

I found it again, my home

i love coming across playable arcade games inside other games.
the example that comes to mind is Klungo's Arcade in Banjo Kazooie: Nuts & Bolts.

Fishing game in Ocarina of Time

Once (I liked it so much I saw it twice...my apologies to Ted Lasso)

I don't know if that fits the definition, but it's an album being written within a movie which sounds like "media within media" to me.

The girl who looked up is a story told within Brandon Sanderson's Stormlight Archive series. It's told very well, and actually told twice.

There are other stories told to different characters in times of need, but I liked that one the most.

GTA 4 in game TV series.

Carl's lock.

Sweet Bro and Hella Jeff

The "Lucky Earther" mini documentary in S06 of the Expanse.

Who Dat Ninja

Inside the 90s anime Martian Successor Nadesico, there is a 70s-era anime called Gekiganger III. Characters various levels of knowledgeable about, or fans of, the show. A single OVA episode was released in addition to the clips from within Nadesico.

This may not fit perfectly into the that category but i think it's cool how Lazarus Jones exists in the media in some capacity in most of the GTA games. One of the best threads of continuity throughout the series.

seinfeldvision

Station eleven

Steel samurai from the ace attorney series.

King's Misery when there are several bits from the novel the writer is making to please his capturer seems like a classic at this point. More classic is the trope of Necronomicon-alikes in lovecraftian stories. But I prefer The Book of Mindfulness that a character from the Black Book sitcom accidentially swallowed and became a saint, even god-like figure.

Dhalgren?

The Mr. Frog show in Smiling Friends.

The movie Twins starring Danny Divito and Arnold Schwarzenegger plays at the drive in the anime Blood Blockade Battlefront

Camp Pining Hearts from Steven Universe. Does Fiona and Cake from Adventure Time count?

Fillory in the magicians trilogy by Lev Grossman is fantastic.

That Baywatch show on friends looks good.

"Always running"