In the Old Testament, all capital “LORD” is used when the English translators were translating the Tetragrammaton (the most holy name of God that was not spelled out entirely, but only using the four letters “YHWH”). That is the reason for the all caps emphasis (such as Genesis 18:1).
In the Old Testament, when the translators use the English “Lord” (only capitalizing the first letter) they are translating the word “adonay” or “adonai” in reference to God (like in Genesis 18:3).
Elsewhere, however, the translators have translated “adonay” as “lord” (not capitalizing anything). They do it like that because the word “adonay” does not necessarily refer to God; it can merely refer to someone in authority (such Abraham as Genesis 18:12).
In the Greek New Testament, there is no Tetragrammaton (YHWH), and that is why there is no English translation “LORD.” The Greek word used for God is “theos,” and the New Testament writers also followed Jesus’ lead by referring to God as “patēr” (meaning Father)
Also, the Greek word for “lord” is “kyrios” (pronounced very much like “curious”), so each time you see “Lord” or “lord” in the NT that is the word the translators are putting into English.
Most interestingly, the New Testament authors largely used the Greek translation of the Old Testament (the septuagint) when they quoted from the Old Testament. The Tetragrammaton was translated into Greek (in the septuagint) as “kyrios” or “lord,” just the same as “adonay.”
So, you won’t find any “LORD” in the New Testament because the authors were generally quoting from the Old Testament text which did not include the Tetragrammaton. But the same LORD of the Old Testament is the Lord of the New, and His attributes remain unchanged throughout the entire text of the Bible.