(Kingdom of Arcadia, Ooo)

In the Castle, we see Jaune & His Wife, Marceline, sitting out In the sun as they are watching their daughter, Mary, playing with her doll, which resembles Marceline's old favorite doll, Hambo, but the color is yellow instead of pink

As for why Marceline is in the sun and not burning like before, it's because Princess Bubblegum made a new antidote that gave Marceline the ability to go into the sun and still be able to have her vampire powers, and that was after Marceline & Jaune's wedding 8 years ago

Jaune: I'm so glad I met you, Marcy.

Marceline laid her head on Jaune's shoulder while smiling

Marceline: I'm glad too.

Then Mary ran up to her parents while holding her doll with a cute smile on her face

Mary: (giggles) I'm ready to go back inside.

Jaune & Marceline smiled at their daughter's cuteness as Marceline stood up and picked Mary up as Jaune & his family walked together and went back inside the castle, hoping nothing or no one could ruin this

(Old Atlas, Remnant)

Near the ruins of what used to be the Kingdom of Atlas, we see Teams RWBY & JNPR arrive at an old Atlas research facility and went inside of said facility as both teams begin searching for anything that can lead them to where Jaune is at now after remembering what the spirit in the Relic of Knowledge told them about where Jaune arrived at 8 years ago

(Flashback)

Ruby: ... what happened to Jaune?

Jinn giggled and began to tell the teams what happened to Jaune via Flashbacks, starting with Jaune arriving in Marceline's house

Jinn: after Jaune was betrayed by everyone, including his own family, he was sent to another world. There, he met a young Vampire girl named Marceline.

the team's surroundings then changed from Marceline's house to Jaune & Marceline sitting on the couch watching a movie

Jinn: they started out as roommates and eventually, after fighting in a battle and saved Marceline's world from the chaos god known as Golb...

their surroundings changed again to Jaune & Marceline embracing and kissing passionately

Jinn: they fell in love.

hearing this made Pyrrha's heart shatter to a million pieces

Pyrrha: WHAT?!

Jinn: and after that, they got married and formed their own kingdom which they called ''Arcadia'', in honor of the dream his grandparents had and told stories about before they died years ago

hearing this made June sad and feel regretful for not only supporting her former twin but also not being there when he founded her grandparents' dream kingdom

Jinn: and one more thing, the world Jaune is in now is known as Ooo.

then she disappeared back into her relic and team RWBY & JNPR's surroundings went back to normal

(Flashback end)

June pulled out a picture of her and Jaune as 8-year-olds, which made her shed a tear from her right eye

June: please forgive me, Jaune.

she put her picture back and resumed searching through the facility with her teammates and team RWBY

and once they find what they're all looking for and arrive in Ooo, they and the Arc & Cotta Arc families will certainly not feel welcomed once they reunite with Jaune later on