The only death I'm aware of is a spiritual death, i.e. sin. Sure, your physical body will wear out and it will go, but the real you will persist. When God created you, at that point, you exist, no different than God exists. The question is... Is where will you exist once the flesh is gone.
The 144,000 people are the 12 tribes of Israel, 12,000 each tribe who are saved during the end times.
A bit of history here:
So, Joshua is tired of fighting. He doesn't complete God's prime directive, wipe out the Canaanites. He dismantles the Canaanites as a functioning society but leaves enclaves of people and giants. These Canaanites later become the Phoenicians. Next King Solomon takes over. For a wise man, he engages in much foolishness, such as learning sorcery, creating division among the 12 tribes, and eventually becoming a tyrant. During Solomon's reign tribes Judah and Benjamin move to the south of the region. Solomon invests a great deal of time, energy and money in this region building up his legacy. This is done at the expense of the North Region, with slave labor and heavy taxation. When Solomon dies, his son takes over. He's just as bad as his father. The 10 tribes in the north plead with him to change policy and lighten up. Instead, Solomon's son doubles down and demands even more from the north. This in turn causes a split from the tribes where the Northern Kingdom establishes its independence, and the Southern Kingdom (Judah and Benajmin) continues in the splendor of Solomon's legacy.
Later the Assyrians invade the Northern Kingdom. The outcome is that the Assyrians won by conquest. However, the whereabouts of the 10 tribes is unknown. Did they flee? Were they all killed? Where they conquered and the eventually assimilated? So, the 10 tribes of the North are out of the picture, gone!
Later again, the Babylonians invade the south. They are merciful compared the Assyrians. Instead of scorch earth warfare, the Babylonians take tribes Judah and Benajmin as slaves back to Babylon. It is here over time that Talmud is authored into two version: Babylonian and Jewish. Eventually tribes Judah and Benajmin are able to leave Babylon and return back to the Southern Region where they came from. When they arrive, they discover that their cousins, the Edomites have successfully occupied the region and developed quite a civilization. This causes both conflict and opportunities for cooperation. Tribe Judah somehow manages to convince the Edomites to convert to Judaism. However, over time the Edomites remained the dominate culture and tribes Judah and Benjamin were assimilated (or "faded away" metaphorically). In the 21st century the people that live and populate the country Israel are not from the tribes of Judah or Benajmin (or the other 10 tribes). They are Edomites. Their ancestors converted to Judaism.
At this point in time the 12 tribes are "missing in action". That is until the end times where the people, whoever they are and wherever they are, realize their true identity and repent. When they repent the 12 tribes return back into the scene where God saves them. They are the 144,000.
These 144,000 are not the only people saved. You have countless millions of people prior to the end times who accepted by faith that Jesus Christ is their Lord and Savior. These people will go to heaven. It's God's promise. God never fails on his promise.
