The Gaza Strip:
The Gaza Strip is a strip of land in the southwest corner of what was once Palestine. Palestinian Arabs fled to Egypt-controlled Gaza after the 1948 War, where they remained until the 1967 War, when Israel took over from Egypt. For about ten years, the Palestinians in Gaza were under the Palestinian Authority with limited self-rule, according to the Oslo Peace Accords with Israel and the PLO. In 2005, Israel withdrew from Gaza and destroyed Jewish settlements. A year later, Hamas took control after winning the Gaza elections against the PLO. They haven't had any elections since. Israel maintains control of the airspace and Mediterranean Sea and continues a blockade, along with Egypt, of the territory.
It is important to note that Hamas maintains Gaza not as a city for its citizens but as a giant military compound with homes, schools, and hospitals, along with the people in them, as shields. Homes also house bomb-making facilities, guns, and ammunition, with children usually near by. Hospitals house military equipment as well and are active staging areas for attacks. Even their military command centers are housed beneath hospitals, schools, and the like. The entirety of Gaza is a loaded gun aimed at Israel.
Hamas spends the billions of dollars being collected from Qatar via the UN, Iran, and from donors around the world, including the USA. Like most terrorist groups, Hamas has had to develop diversified sources of income. They maintain a presence in South America, participating in drug, people, and arms trafficking, as well as money laundering. The money of any significance is not spent on schools, food, or medical equipment but on tunnels to move troops and materiel, the manufacture of rocket-propelled grenades, bombs, guns, ammunition, rockets, and all sorts of military equipment. Besides tunnels, ambulances are used to move troops on the surface so they are not a target. And, of course, their arrangement has made Hamas leaders very rich.