Black Mirror. News tickers and headlines discuss other world-building stories from different episodes.

The poems of Thomas Zane in the old gods of asgard concert in the book about max Payne in the video game Allen wake in the video game control in the amazing TV series the threshold kids in that song the janitor is singing.

... Pretty sure that's the continuity.

Wheel! Of! Fish!

Los Dias Y Las Noches de Monsignor Martinez in King of the Hill.

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.

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.

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

• 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.

GTA 4 in game TV series.

Carl's lock.

Sweet Bro and Hella Jeff

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

That Baywatch show on friends looks good.

"Always running"

Steel samurai from the ace attorney series.

I’m surprised and saddened for it to have taken so long for someone (in this case, me) to have mentioned Astronaut Dolphin Detective from Mr Pickles.

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

The Mr. Frog show in Smiling Friends.

seinfeldvision

Dhalgren?

I seem to remember one of the GTA games (maybe San Andreas?) had a stand up set from I think Ricky Gervais as something that would play on the TVs.

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.

Who Dat Ninja

Fillory in the magicians trilogy by Lev Grossman is fantastic.

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.

Station eleven