@neologist,
This is a long story... I have assembled some wiki + other material to try and tell it.
We all know the Hebrew word "El" is translated as "God", as its plural "Elohim", which is generally understood in the Bible as a plural of majesty for a unique God. However, sometimes the biblical "elohim" means a real plural: "the gods", and/or is interpreted as "angels". The word "El" appears in many Jewish names like Emmanuel, Samuel, or Daniel. We also hear it in the Arabic "Allah".
So the modern understanding of the term "el" is "God" or "god". But there is evidence that El was originally the name of the main Canaanite god.
In 1929, archeologists discovered many engraved clay tablets in the digs of an ancient Canaanite city, Ugarit, in present-day Syria. The so-called ugaritic tablets describe in rich detail the pagan background from which the religion of Israel emerged. They depict El as the sovereign and pan-creator deity of the Canaanite pantheon. He is referred to as “Father of the gods,” “the eternal King,” and “Creator of all living beings.”
El was also the god of the patriarchs. The Pentateuch identifies Him as the deity to whom the patriarch built shrines and altars. Genesis 33:20 says that Jacob builds an altar in Shechem, and dedicates it to “El, god of Israel” (’el ’elohe yišra’el ). Judges 9:46 speaks of the shrine of “El of the covenant" in Shechem. Genesis 14:18-22 speaks of “El the most high,” of whom the Canaanite Melchizedek is priest at Jerusalem.
Gen 35:11: God said to him, “I am El Shaddai [...] 15 Jacob named the place Bethel ["house of El"] where God spoke to him.
The meaning and etymology of El Shaddai are unclear, perhaps "El of the Mountain" based on an Assyrian cognate but shaky. Many a pagan god dwells on mountains... Anyway, it's usually translated as "God Almighty" in English but that's a rationalization. No one has a clue what the Hebrew text really means. It must be a local epithet of El.
Jacob has another encounter with El at Penuel, which is so named because Jacob sees El face-to-face (32:31). Moreover, Isaac blesses Jacob through El Shaddai (28:3), and likewise Jacob blesses Joseph “by El of your fathers” (49:25).
So in the polytheist matrice that gave birth to Judaism, before "El" became synonymous with "God" in the biblical narrative, El was the name of the big boss in the sky.
YHWH is obviously a different name. Have you ever wondered why God would feel the need to change His name in the middle of the Bible like that? What kind of a god needs an alias?
A composite god maybe, an amalgamation of 2 different gods... Thus the idea that YHWH was originally another god than El. A lesser god, perhaps his son, or simply a god from another corner of the Middle East: Midian (Sinai and/Negev), where Moses lives in exile and sees the burning bush. In the wilderness of Midian, the voice from the bush introduces Himself to Moses as:
Exodus 6:3: and I appeared unto Abraham, unto Isaac, and unto Jacob, as El Saddai, but by my name YHWH I didn't make me known to them.
So YHWH is introduced in the narrative as just an alias. And there are many other quotes underlining that YHWH and El are one and the same. However, Deuteronomy 32:8-9 is one of those rare biblical passages that seemingly preserves a vestige of an earlier period in proto-Israelite religion where El and Yahweh were still depicted as separate deities:
When the Most High (’elyôn) gave to the nations their inheritance, when he separated humanity, he fixed the boundaries of the peoples according to the number of divine beings. For Yahweh’s portion is his people, Jacob his allotted heritage.
We could discuss the translation, but many scholars see here the two deities as clearly separate in the original text. Yahweh is merely one of the gods inheriting a Canaanite nation's care from El: the nation of Israel. This tradition probably comes from older Canaanite lore.
At some point of the evolution and demographic growth of the Canaanite societies, the ancient cult of El (which apparently all Canaanite shared in) was superseded by cults to lesser deities: El's sons and daughters. Each of the Canaanite nation picked one or two national gods (2 because these Canaanite gods tended to be married). Ball is one of them, YHWH another. Maybe it was more convenient in warfare, to be able to oppose your god to my god... In any case, Israel's lesser tutelar god was Yahweh.
Similarly, in another older tradition now preserved in Numbers 21:29, the god Chemosh is assigned to the people of Moab. The Mesha stele, a Moabite engraving now at the British Museum, confirms this:
The Mesha Stele (also known as the "Moabite Stone") is a stele (inscribed stone) set up around 840 BCE by King Mesha of Moab (a kingdom located in modern Jordan). Mesha tells how Kemosh, the God of Moab, had been angry with his people and had allowed them to be subjugated to Israel, but at length Kemosh returned and assisted Mesha to throw off the yoke of Israel and restore the lands of Moab. The stone bears the earliest certain extra-biblical reference to the Israelite God Yahweh.
Other biblical passages reaffirm this archaic view of Yahweh as a god in El’s council. Psalm 82:1 speaks of the “assembly of El,” Psalm 29:1 enjoins “the sons of El” to worship Yahweh, and Psalm 89:6-7 lists Yahweh among El’s divine council.
Psalm 89:6: Is there any in the sky who could compare to YHWH?
Who among the gods is equal to YHWH?
7 El is respected in the council of the gods (elohim);
El is awesome and revered more than all those around him.
Note that most English translations have 'the Lord' for YHWH and 'God' for El. But the text may be understood as speaking of two distinct deities, as in Deut. 32:8-9.
Thus there seems to be ample evidence in the biblical record to support the claim that as Yahweh become the supreme national deity of the Israelites, he began to usurp the imagery, epithets, and old cultic centers of the god El. This process of assimilation even morphed the linguistic meaning of the name El, which later came to mean simply “god,” so that Yahweh was then directly identified as ’el—thus Joshua 22:22: “the god of gods is Yahweh” (’el ’elohim yhwh).