Access Modifiers in C#: public, private, protected, internal — what is the difference and when to use what?

Hello everyone! I just started learning C# and writing my first classes. I stumbled upon access modifiers. I kind of understand that `public` is "for everyone", `private` is "only inside the class", and `protected` is "for inheritors". But in practice, I get confused. Here is an example. I have a class `BankAccount`:

class BankAccount
{
    public string AccountNumber { get; set; }
    private decimal balance = 0;
    protected decimal interestRate = 0.05m;

    public void Deposit(decimal amount)
    {
        balance += amount;
    }
}

Then I create a class `SavingsAccount` that inherits from `BankAccount`:

class SavingsAccount : BankAccount
{
    public void ApplyInterest()
    {
        // Here is the problem: I can use interestRate (protected),
        // but I cannot use balance (private).
        // This is clear. But what if I want to give access to balance
        // only to inheritors, but not to everyone else?