When you type let's say 10 + 20 and you'd like to divide the sum by another number, you have the option to only use the closing parentheses and the app will automatically assume (or insert upon pressing equals) the opening parentheses for you. Alternatively you can also tap and hold the opening number (1) to auto-insert a parentheses as you type and then again tap and hold the last zero (0) to auto-close any existing parenthesis.
Whenever you start an expression with opening parenthesis i.e. 1/2 x (3 + (4 - 6 ... the app will automatically evaluate the solution following the standard math order of operations and insert all closing parenthesis again when or if you press equals "="
The same goes for opening parenthesis, if you decide to only use the closing ones and do not wish to count the necessary number of opening parenthesis i.e. 3 + 4) x 2 - 6) x 3 - 8 ~ CALC will follow the standard math order and instantly calculate, respectively auto-insert any opening parenthesis upon pressing equals if you so choose.
The order of operations math rule - where multiplications and division take precedence - is a basic principle that dates waaayyy back. If you need a quick refresher, Wikipedia has a short summary on it: https://en.wikipedia.org/wiki/Order_of_operations