I can’t create an infinite loop in the same loop. There are no errors, but the program just skips some of the code starting from the second while.

while True:
        print("Enter'Add'  Add  ")
        user_input = input(": ")
        if user_input == "Add" or "Add":
            while True:
                numb1 = float(input("Enter : "))
                numb2 = float(input("Enter : "))
                print(str(numb1), str(numb2))

Answer 1, authority 100%

>>> while True:
...     print("Enter'Add'  Add  ")
...     user_input = input(": ")
...     if user_input.lower() == "Add":
...         while True:
...             numb1 = float(input("Enter : "))
...             numb2 = float(input("Enter : "))
...             print(str(numb1), str(numb2))
...
Enter'Add'  Add  
: Add
Enter : 10
Enter : 5
10.0 5.0
Enter : 15
Enter : 3
15.0 3.0
Enter : 123
Enter : 10
123.0 10.0
Enter :

The program exactly follows the logic that you put into it, if you correct the incorrectly composed condition.
What comes after the cycle is not skipped – the queue simply does not reach it. If you need any other code to be executed after the loop, it makes sense to exit the loop:

>>> while True:
...     print("Enter'Add'  Add  ")
...     user_input = input(": ")
...     if user_input.lower() == "Add":
...         while True:
...             numb1 = float(input("Enter : "))
...             numb2 = float(input("Enter : "))
...             print(str(numb1), str(numb2))
...             if numb1 == numb2: # the condition under which the loop ends. I used the condition of equality of two numbers, you may have it different
...                 break
...         print('***** The endless loop is over, the code is executed further')
...
Enter'Add'  Add  
: Add
Enter : 3
Enter : 2
3.0 2.0
Enter : 3
Enter : 5
3.0 5.0
Enter : 5
Enter : 5
5.0 5.0
***** The endless loop is over, the code is executed further
Enter'Add'  Add  
:

Regarding the error in your code:

user_input = input()
if user_input == "Add" or "Add":
    print('This condition is always met!')

You can check it yourself. The fact is that, taking into account the priorities of operations, the condition looks like this:

if (user_input == "Add") or ("Add"):
    pass # do something

Boolean expression user_input == "Add"can take either a true value or a false one, but the boolean value of the line "Add"is always true, because any non-empty string has a true boolean representation. If you want to use exactly this approach to the solution, and not by converting to lower case, the condition should be of the form:

if user_input in ("Add", "Add"):
    pass # do something