why doc is not strightforward

What is not straightforward?

i dont know where should start from for modeling and wrditing statuful functions

You mean https://nightlies.apache.org/flink/flink-statefun-docs-master/ ?

how to qccess old data when new event has come

The documentation is basically a reference and doesn't contain many howto's or examples, I agree on that. In the end, that's also a matter of volunteers helping out to provide better documentation

i want create matching engine connecting drivers to passenger or buyer to sellers how can i qccess them

I think you will get the best answer if you post what you've tried and what error you're getting. Right now, you're basically asking someone to write an application for you. Given that this community runs on volunteers, I'm afraid that you will not get many replies on your original question

for example see nextjs.org as a best documenting we find anything we want

Yes, but nextjs.org is a commercial organization, not a free and voluntary software project like Apache Flink

yes you right this wnats many hours to documenting

I understand that you're feeling like that, but I don't think this community isn't trying to help. However, we can't help everyone

nice