Custom php api for huge known national software. No frameworks. Not following mvc structure. No decent documentation. Im the new dev and like how the fuck should i catch up before they think im not productive

Oh dear...

Try to ask more. Ask the poeple that knows. Don't be afraid of showing your weaknesses, but don't let them be forever. Well i guess you are new. They will probably take it easy on you at first so don't be afraid to ask more often than you usually do. Don't worry with time you will see there is more people like you in the office than you think. So hold on in there body and keep up the learning. Even if you got fired ( worst case ) it will happen after 1 or 2 month from the starting date. So it is still a win situation if you learned something from them.