As for your own DIY process, Dishwashing soap works the best. Try it out yourself on a Glass of drinking water to test it out. What ever you use on it and it gives you that clean glass feeling with your eyes closed and makes the squeaky sound. That’s the one you want to have to mix it with the water in the windshield washer tank.
When you go to buy the solution of the shelf, usually they are just glass cleaners which are diluted with water and put into bottles for you to pickup, pour and use.
And if your cleaning your windshield from inside and it has that oily reflective white Smokey residue which is very Annoying at night. It doesn’t give you a good viewing vision every time a light hits from the opposite direction. This usually is found in car that are of regular smokers, just make sure you use few plys from the kitchen roll instead of newspaper reason being that the ink from the newspaper won’t stick back onto your windshield. Yes the newspaper does the job and you keep cleaning it regularly in a couple of days or once a week. But with the kitchen roll it’s made to absorb the oils and has not ink coming off it. So you tend to have a clean sparkly windshield like those commercial advertisements 😝. AS SHOWN ON TV.