Bitsat is an online exam questions are generated from a large question bank randomly. So how are its previous years question papers available?? every candidate gets different questions right ?

That is absolutely correct. BITSAT papers are generated from a large bank of questions. The manner in which the last year papers are generated or procured to be correct by compiling all the questions from various tudents who just gave the paper or literally sending in volunteers whose sole purpose is to sit for tha paper and remember the questions which came online and reproduce the questions later.
With 4-5 volunteers like these I can have 4-5 full papers, because my volunteers only gave the paper such that he/she can get questions for me.
If I am targetting a larger bank , then apart from this I would also survey the genuine students who just came out of the center and ask for the questions they remember which came in question paper.