It starts with your threads. Most compatible and most seen fittings are G1/4 threads. Threads are the fittings themselves; you have barbed and compression fittings, though in my opinion compression fittings feels more secure to me. Then you have the tubing diameter - also pronounced as "OD" (outer diameters) and "ID" (inner diameter). Every tubing size fits on different thread fittings. Most common seen are "3/8, 1/2, and 7/16". The G1/4 fits in this case with the 3/8 size tubing size.
You dont have to buy anything to change degrees of tubings - but extenders. Always keep looking for the same threads(fitting measure) Lets say if you consider getting G1/4 fittings with 3/8 OD tubing. You just order all the fittings you need(always keep in mind 2 fittings for 1 component - in and out). For bendings and heights you have special extenders to make the right angle.
EU/US universal fittings:
10/13cm (3/8 - 1/2)
10/16cm (3/8 - 5/2)
12/16cm (7/16 - 5/8
13/19cm (1/2 - 3/4)
This image will help you clear information and helps you decide what fittings and tubings to go with:
https://shop.ekwb.com/media/wysiwyg/fitting_and_tubing.jpg
I hope you found the solution to your answer and wish you the best of luck building a water cooling system.