Documentation

The PEP 8 style guide recommends adding docstrings for functions. You can convert the function comment:

def getExpression(level, operator):
    # Generate operands based on the level

into a docstring:

def getExpression(level, operator):
    """ Generate operands based on the level """

It would also be good to add details about the input types and return types.

Naming

PEP-8 recommends snake_case for function names.

getExpression would be get_expression

Validation

Unless the code that calls your function makes sure that the inputs are valid, consider adding some simple checking to the function. For example, you should think about how to handle unexpected values passed to the level parameter. From the code, you expect it to be an integer between 1 and 5. If the function is passed an integer outside that range (like 6), the code exits with an error. The same is true if passed a floating-point value (2.1) or a string ("hello").

DRY

There is a lot of repeated code. For example, for the addition operator, you can take advantage of the relationship between the level and the upper and lower limits of the values passed to randint. For levels 2 and above, you can replace the hard-coded values (10, 99, 100, 999, etc.) with function calls, where you declare new functions:

def lower_limit(level):
    """ Return lower limit as a power of 10 """
    return 10**(level - 1)

def upper_limit(level):
    """ Return lower limit as a power of 10, minus 1 """
    return (10**level) - 1

This reduces the addition code to:

if operator == "+":
    # Addition
    if level == 1:
        num1 = random.randint(1, 9)  # Single digit operand
        num2 = random.randint(1, 9)
    else:
        num1 = random.randint(lower_limit(level), upper_limit(level))
        num2 = random.randint(lower_limit(level), upper_limit(level))
    
    # Return the expression and result
    result = num1 + num2
    expression = f"{num1} + {num2} = "

This eliminates much of the repetition, and it allows the code to scale to more values of level. These functions can also be used with the other operators.