How to making a complex English sentence
I am quite amused by how to structure a complex English sentence.

here is some example:
1. A jury is required in many court cases ranging from the most serious criminal offences to some civil cases.
2. In contrast to a "pull" system, in which units in the field would request the supplies the needed, the push system required supply experts back in the United States to decide what to send.

How to create such a long sentence without compromise on the grammar and the meaning. Any tips?

Question

How to making a complex English sentence
I am quite amused by how to structure a complex English  sentence.

here is some example:
1. A jury is required in many court cases ranging from the most serious criminal offences to some civil cases.
2. In contrast to a "pull" system, in which units in the field would request the supplies the needed, the push system required supply experts back in the United States to decide what to send.

How to create such a long sentence without compromise on the grammar and the meaning. Any tips?

Answer

Hello,
The short answer: punctuation.
In your first example: 
"A jury is required in many court cases, ranging from the most serious criminal offences to some civil cases."
Because the second part after the comma can not stand on its own as a sentence, a comma is used.
A semicolon ( ; ) would be used when joining two parts of a sentence, if both parts could stand on their own as sentences.

Answer

It will get easier to make complex sentences as you learn more about English grammar. However, unless you are doing some sort of professional writing for work, people don't really like to use sentences like that. They aren't good for quick and effective communication, so in daily life people don't really talk like that.