Can someone install spyware into my iPhone?

I had an iPhone 6 at the time, I no longer use it due to the screen messing up since I dropped it without a case a few times. On one occasion some friends of mine had the finger print access to my iPhone and while I was asleep they went on my phone. At the time it seemed harmless, they changed my wallpaper, they went through my apps (the apps were still open whenever you double tap to clear the apps you had been using), i remember one of them sent themselves something (which I have forgotten now what it was), I think they were just browsing? I had just gotten it so I didn't really have much on there. Recently I have started to worry, what if they installed something where they could track me or listen to my conversations or all of that stuff that spyware does. Since I don't have my phone and since I wouldn't bring that up to my friends since that's a pretty heavy accusation when I don't really have proof except that they had access for hours without me knowing. Would I have noticed anything different? Is it even possible to do on an iPhone 6 roughly three years ago?

Could I switch to a different phone carrier and still keep my original phone number?

I currently have boost mobile but I want to switch to a different phone Carrier in order to get a new iPhone? I currently have an 8 and have boost mobile? What carriers would accept me when transferring? I heard that t mobile runs on a different frequency. Also could I still keep my existing files such my contacts when switching to a different carrier?

Is snapchat hackable?

Our child who is 12 has been using snapchat and has recently been in trouble for not using it correctly. He was caught smoking and drinking and sending it to friends. Were planning on trying to monitor his snapchat and I can only do so much when I'm not around so i want to know is it even hackable? I know some coding and am slightly knowledgeable with technology. Has anyone tried it remotely? What was your expeience with it? Are the apps worth it? How do I jailbreak his iphone or root his android?

My cousin is a brat?

So I have a cousin from Vietnam and I used to hang out with her a lot when I went and visit Vietnam for 6 weeks and she was one of the sweetest people ever. Fast forward to 2 years later, we're both 16 now and I just found out online that she's dating a 22-year-old guy from overseas. I told my mom about it and she told my aunt (my cousin mom) about it. Her mom freaked out and didn't know she's dating a guy 6 years older than her. I guess it's kind of common over there to date someone with a big gap age, but she's barely a sophomore in high school and he's gonna be graduating from college. She's also been calling my mom asking her for some money so she could buy a motorbike to drive herself to school and she didn't want to use her mom old motorbike. My mom wouldn't give her any money and a couple months later, my cousin then ask my mom for a new iPhone X because she got this new job that pays her to take pictures of the store product and post them online in their website and her iPhone 6 camera is broken and she can't use it. My mom declines her and told her to ask her own parents for one instead. She's also been sneaking out at night drinking and would come home drunk. Now she's saying that after she finishes school she wants to marry her boyfriend and have kids or if her mom lets her, she wants to drop off of school and marry her boyfriend and have kids. I don't know how I feel about her, she's also been asking me to buy her new clothes and makeup overseas too.

Added (1). I just found out they only dated for three months and he bought an airplane ticket to see her. She's now going to me asking me to get her stuff that cost so much money and I don't want to waste that much money I've been working for (I'm saving up money for my Greece trip). If it wasn't expensive I wouldn't mind but ooofff she wants Channel, Champion, Supreme, etc.

Am I considered 'spoiled?

Hi I'm 18, a guy. I have a job and I bought JBL headphones, an Gold iPhone XS Max, and a 2018 MacBook Air. Does that count as spoiled? I mean, I worked hard for them. Those apple products aren't cheap!

Did my paranoid Boyfriend track my IPhone?

I went out of town this weekend with family. And he suddenly won't answer my texts/ phone goes right to voicemail (doesn't ring)
He's the type who's always paranoid.
Is it possible he could track my location? We both have iPhones. My phone is always locked.

IMessage turned from blue to green, did I get blocked?

I sent a message from my iPhone To my friend who also has an iPhone, and texts between us are always blue, this time when I texted him it was blue as usual after like 3 min I opened the message again to find it green ( sent as a text) did he block me! Is there another way to know without trying to call him? Maybe sending another follow up message will help me figure out, if he blocked me or not, thanks in advance.