What the heck?
'It's up to the DM, because invisible means something specific'?
If invisible means something specific then how it works is not up to the DM.
It has a specific RAW effect. The Invisible condition in the 2024 rules, I mean. It does four things, two of which affect attacks, one of which affects targetting with spells and other effects, one of which affects Initiative rolls. Those things don't necessarily come up in play while exploring a dungeon in abstract time, so if a player is asking to Hide, I have to interpret what it is they want out of the action. What are they trying to achieve? What do they want to take the Hide action for? If it's just "to become invisible", then I may have to explain that the Invisible
condition, which Hide grants, may not achieve what they're imagining it does.
Because, again, and I must have said this eighteen million times in this thread now, the Invisible condition is not
necessarily being completely invisible in the sense of putting on the One Ring. It's just an in-game effect, with quite limited rules. So, in fact, if everyone Hides and becomes Invisible, the monsters might
not just ignore them, depending on the situation. I might decide we roll Initiative and the monsters Search for a bit and try to spot the party, if I think it would be worthwhile. Or I might just nod it through, since they made the effort to conceal themselves while exploring (but, again, I'd have to determine how possible that is based on their actions).