My fiance's complicated relationship with her male best friend, what's their thing?

Hey, so... I'm 23 (M), and I got engaged (arrange marriage) to my fiancée 2 years ago. She's 21 (F).

So, she had a boyfriend in 12th grade. They dated for a month or two and broke up because her family found out, but that’s not the issue.

She has a male best friend (21M). They've been friends since 11th grade. He says he's hung up on his ex, etc., and she and he talk a lot. I saw their chats, and he calls her "Cutie, Madam ji," etc. They say "miss you" and "good night ❤😊," which I guess is normal. We had a fight because of him too, and she got angry because I told the bestie to make some distance. We weren't talking for a while. They had a fight too, and she said sorry for making you this attached, and he said, "You don't give me time," etc., and they didn't talk for a week. He was sending her reels, and she said, "You are making it seem like I'm breaking up with my boyfriend." They started talking again, and they went for coffee twice. On her birthday, he gave her a Kinder Joy, and their group went to a café.

Last night, I saw her saying, "The guy I chose in the past and the guy my family chose (me) are both chutiya 🙂."

He said something, I guess quoting lines from the Laila Majnu movie: "Let's go behind mountains." She said, "Haaye ❤, chal." He said, "Grab my hand." She said, "Let's go." He said, "We both are broke though ☠️."

He once complimented her, and she put it on her status. They use this "😏" a lot. He writes poetry for her and stuff.

She says, "You are my best friend," etc., and tells me this too. Sometimes she talks alot and sometimes she ignores him

I don't know how to deal with this mess. I can't break the engagement. What should I do?

What's this bestie thing? Is it normal? Am I overthinking?

QUESTION:- WHAT'S THEIR THING?! WHAT SHE FEELS FOR HIM!?