How do you calculate the running time using Big-O notatation?

I’m still new to Data Structure and Algorithm and therefore I would like to ease my doubts. I’m required to find the Big-O running time of myMethod():

 Static int doIt(int n) {
           for ?←1 ?? 100 do    -> 100
             for ?←1 ?? ? do    -> n x 100
                 ?←1;?←?       -> 1
           while ?<? do       -> lg n
          ?←(?+?)/2           -> lg n/2
        end while 
        end for 
        end for
    
    }

 static int myMethod(int n) {
                  ? ←1
                  ?ℎ??? (? < ?) {          -> lg n
                        ????(?)            -> ?
                         ?←?×2             -> lg n
        }
            ?????? 1; 

}

For each of the line I have inserted the answer I think would be a logical value to put e.g "-> 100". However I am not confident with the answer. These are the following questions that I would like to ask as well:

1. Does all while loop have a value of lg n?

2. If the doIt(n) method does not have a return statement, do we still add in part of calculation?

3. For the line with m <- (m+j)/2, the value I have inputted is lg n /2. Is that correct or is it some other value?

4. Whenever multiplying by 2 or dividing by 2 is it always lg 2? what if n is multiplied by 3?

Thank You