Dunno why this thread is filled with people equating Hezbollah and Hamas, but they are different organizations representing different groups of people. Israel attacked Hezbollah with exploding pagers, not Hamas.
It's a pretty big and important distinction, because this attack by Israel was only tangentially related to their genocide in Palestine. It's further evidence of their expansionist goals as they continue a campaign of terror against other countries in the region.
Palestinians stopped using cell phones because they could be geolocated by triangulation of cell towers. They began using pagers.
Prerequisite knowledge: If a lithium ion battery is exposed to air it'll burn hot and fast. If it's severely overcharged the protective case can be breached. But, it wouldn't take much to breach the case on purpose.
Israel surreptitiously sold the Palestinians pagers rigged to use the lithium ion battery as an improvised explosive device. Then, they detonated them all at once.
Editorial:
It's pretty fucked. But, they're killing much for indiscriminately already, targeting hospitals and schools. The OP exaggerates only the escalation to chemical weapons. Israel is a tool of the US's most successful proxy war.