Is this for your Apple Account or for your T-Mobile account?
Security questions for your Apple Account are meant to keep your Apple Account safe, as only the user would know the answers. Security questions are designed to be memorable to you, but hard for anyone else to guess.
An Apple Account is potentially much more than just for a streaming service, also hold sensitive private data and even account balance, so security is very important here.
If you are setting up your Apple Account for the first time, then you may be presented with questions for which you are asked to provide answers (anything goes as long as you can remember what you filled in), so that future asking may be compared to these answers.
Not knowing the answers when asked may lock you out of your Apple Account.
A better way is two-factor authentication, where an Apple device or trusted phone number is used to send codes to for authentication, so you don’t have to remember the answers to questions that you would have previously provided.
You may need to fill in the answers before you can set different questions or switch to two-factor authentication. Else anyone could take over your account, and you wouldn’t want that.
Apple Account - Official Apple Support
Get support - Apple Support
Change your Apple Account security questions - Apple Support
Two-factor authentication for Apple Account - Apple Support
Get a verification code and sign in with two-factor authentication - Apple Support