Until you know the internals of SAP ;). They have/had an own programming language (ABAP) which is the worst combination of BASIC and SQL you could imagine. Do not know the status in 2020, 2007 it was still a combination of ABAP and Java.
Interestingly, the source code of SAP is openly readable (once you have their software installed).
SAP is effectively open source, IIRC there were/still are options to install local development stacks for free... but without an army of experienced consultants it's gonna be useless for you and they won't touch a pirated/cloned system with a 10 feet pole.
Interestingly, the source code of SAP is openly readable (once you have their software installed).