This isn't really a note from your big sis, but I'm imagining that she would write this note to you. (This is actually her dad, pretending to write to you in her voice).
As parents, when raising our first kid, we didn't really have a "strategy" or "approach" to kids and technology. Like most parents in most areas, you try your best, but make up a lot of stuff as you go. For gadgets, I tried to impart a sense of ownership and responsibility with each device (iPad w/ Wifi, iMac w/ printer w/o internet, etc.).
Here's the note I'd like her to write you:
Today, I got really mad at my iPhone, because I decided to change the passcode. Why? I didn't have a very good reason, maybe because I wanted a passcode my parents didn't have. After changing the passcode I could not get back in. After trying too many times, I was locked out of my phone.
I had to erase my phone. I don't backup my phone very often, eventually I had to restore and old backup, and I lost 1 month of my "life".
Unimportant: whether or not I mis-remembered the new passcode, or Settings froze and messed up. What matters here: Apple doesn't tell you anything about how the phone actually works, which is fine until it doesn't work.
The experience was disgusting. I screamed and cried. I had to let my Dad fix my phone (and I had to wait a whole day for that). My dad says its called "bricking" because it becomes useless. I think it means "I'm gonna throw it thru a window".
What learned, besides the fact you should never change you passcode without first doing a backup, is that you (the next generation) need and deserve excellent documentation. Clear, simple-yet-detailed explanations for how all this junk works.
Okay. She probably wouldn't really write that to you.
Here's what the current Apple documents say:
If you enter the wrong passcode on an iOS device six times in a row, you'll be locked out and a message will say that your device is disabled. Learn what to do.
(Then a picture of a phone that says: iPhone is disabled / try again in 1 minute)
(Then instructions on how to erase your device).
Here's the title: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled
The article assumes that six guesses wrong is the same as being locked out forever. But it isn't.
There are two versions of the screen. After 6 wrong guesses, you get a disabled screen with a timeout. After 10 guesses, you are permanently locked out.
This might sound like a trivial distinction, but it won't be when you are in between your 7th and your 10th guess.
Here's the document I think you deserve:
Your iPhone should always have a passcode. It keeps people out of your phone. You can use Touch ID or Face ID as well, but they require a passcode to work.
If the wrong passcode is entered too many times, your phone will become disabled, and you will have to erase it. If you do not have a backup, you will lose everything and will be like starting over with a new phone.