Accessing data in SwiftData in SwiftUI

I'm trying to understand and learn SwiftData. I want to keep some data in my application. I wanted to use swiftData to learn SwiftData and created a model like this:

import Foundation
import SwiftData

@Model
final class SwiftDataModel {
    
    var height: Optional<Double>
    var heightUnit: Optional<String>
    var weight: Optional<Double>
    var weightUnit: Optional<String>
    var gender: Optional<String>
    var waterGoal: Optional<Double>
    var volumeUnit: Optional<String>
    var activity: Optional<Int>
    var age : Optional<Int>
    var nutritionLog : Optional<Array<Any>>
    
    
    
    init(height: Optional<Double>, heightUnit: Optional<String>, weight: Optional<Double>, weightUnit: Optional<String>, gender: Optional<String>, waterGoal: Optional<Double>, volumeUnit: Optional<String>, activity: Optional<Int>, age: Optional<Int>, nutritionLog: Optional<Array<Any>>) {
        self.height = height
        self.heightUnit = heightUnit
        self.weight = weight
        self.weightUnit = weightUnit
        self.gender = gender
        self.waterGoal = waterGoal
        self.volumeUnit = volumeUnit
        self.activity = activity
        self.age = age
        self.nutritionLog = nutritionLog
    }
}

Maybe I can also add an array like weightlog. As far as I understand swiftData keeps this data in a list. Therefore, I will always need to update index 0 instead of using context.insert(data). Is this true?

My other case is this: After the user logs in, can I check whether the user's age, weight and height information is kept in SwiftData and determine the page to be opened there?

For example :

@main
struct MyApp: App {
    
    var body: some Scene {
        WindowGroup {
            
            if (ViewModel.isLogin() && !ViewModel.userId.isEmpty ) {
                // If height, weight, and age information is available in swiftData TabBarView()
                // else IntroPageView()
                {
            } else {
                LoginView()
 
            }
    
        }
    }
}

Is such a check possible or should I use "User Deafults" or "Core Data" instead of SwiftData in my application?

I'm looking forward to your answers. Thanks in advance

Thank you for all the answers. It did what I wanted and I'm using it efficiently.

First, I updated my model like this:

@Model
final class UserInfoSwiftDataModel {

@Attribute(.unique) var userId : String?
var height: Double?
var heightUnit: String?
var weight: Double?
var weightUnit: String?
var gender: String?
var waterGoal: Double?
var consumedWater = [Water]()
var consumedWaterInt : Double?
var volumeUnit: String?
var activity: Int?
var age : Int?
var nutrition = [Nutrition]()


init(userId: String? = nil, height: Double? = nil, heightUnit: String? = nil, weight: Double? = nil, weightUnit: String? = nil, gender: String? = nil, waterGoal: Double? = nil, consumedWater: [Water] = [Water](), consumedWaterInt: Double? = nil, volumeUnit: String? = nil, activity: Int? = nil, age: Int? = nil, nutrition: [Nutrition] = [Nutrition]()) {
    self.userId = userId
    self.height = height
    self.heightUnit = heightUnit
    self.weight = weight
    self.weightUnit = weightUnit
    self.gender = gender
    self.waterGoal = waterGoal
    self.consumedWater = consumedWater
    self.consumedWaterInt = consumedWaterInt
    self.volumeUnit = volumeUnit
    self.activity = activity
    self.age = age
    self.nutrition = nutrition
}



}

@Model
final class Water {
var date: String
var consumed : Double
var User : UserInfoSwiftDataModel?

init(date: String, consumed: Double, User: UserInfoSwiftDataModel? = 
nil) {
    self.date = date
    self.consumed = consumed
    self.User = User
}
}

@Model
final class Nutrition{
var name: String
var calories: Double
var serving_size_g: Double
var fat_total_g: Double
var fat_saturated_g: Double
var protein_g: Double
var sodium_mg: Int
var potassium_mg: Int
var cholesterol_mg: Int
var carbohydrates_total_g: Double
var fiber_g: Double
var sugar_g: Double
var User : UserInfoSwiftDataModel?

init(name: String, calories: Double, serving_size_g: Double, 
fat_total_g: Double, fat_saturated_g: Double, protein_g: Double, 
sodium_mg: Int, potassium_mg: Int, cholesterol_mg: Int, 
carbohydrates_total_g: Double, fiber_g: Double, sugar_g: Double, User: 
UserInfoSwiftDataModel? = nil) {
    self.name = name
    self.calories = calories
    self.serving_size_g = serving_size_g
    self.fat_total_g = fat_total_g
    self.fat_saturated_g = fat_saturated_g
    self.protein_g = protein_g
    self.sodium_mg = sodium_mg
    self.potassium_mg = potassium_mg
    self.cholesterol_mg = cholesterol_mg
    self.carbohydrates_total_g = carbohydrates_total_g
    self.fiber_g = fiber_g
    self.sugar_g = sugar_g
    self.User = User
}   
}

To stop using Index, I added an id match condition to get the data of the logged in user. This way, I can access and update information in a healthy way. I used it as follows:

@Environment (\.modelContext) private var context
@Query (sort: \UserInfoSwiftDataModel.height, order: .forward) private 
var mySwiftData : [UserInfoSwiftDataModel]
var userId : String // Login/SignUp ekranından geliyor
var profile: UserInfoSwiftDataModel? {
    return mySwiftData.first { $0.userId == userId }
}

Text(" \(String(format: "%.1f", profile?.weight ?? 0)) \. 
(profile?.weightUnit ?? "kg")")

This way, I can easily get my data not only within the list, but everywhere. Thank you for giving a time.