The New Testament is our divinely inspired commentary on the Old Testament. When studying a passage, it’s often helpful to see how other biblical authors understood it. Here is Mark’s use of the Old Testament in his gospel.
A quotation is a word-for-word citation of the original source, often preceded by “it is written” or “saying”.
An allusion is a reference to the original source using the the same key words or thoughts without quoting word for word.
Because the New Testament writers often borrow the language of the Old Testament (allusion) rather than quote verses directly, different scholars suggest different counts for the number of times the New Testament quotes or refers to the Old Testament.
Quotes from the Septuagint (LXX) may have appear to have somewhat different wording than the linked translation of the Hebrew text.