UN seems to believe they're not made up, don't know how they're verifying it

It is definitely true that the numbers are unlikely to be exact. That is both for the reasons others have noted, propaganda is definitely at play on both sides of any conflict. A second reason for the likely imprecise number though, is that many casualties are probably uncountable currently as numerous buildings have collapsed under bombardment. Fully surveying those sites for bodies is difficult with continuing combat in the area. We know the buildings are destroyed due to surveillance and other intelligence sources, but can't yet know how many people were killed and injured during the destruction.

There are also reasons that the UN and other international organisations take the figures seriously. This is primarily because of historical accuracy, this is not the first occasion of strikes in the area and surveys in the aftermath of previous incidents have generally shown reasonable reporting accuracy from the Gazan health authority. Additionally there are other international organisations on the ground and reporting corroborating evidence of mass collateral casualties. UNICEF has supported the reporting on the 3700 children killed so far, inclusive of civilians on both sides. Recently Doctors Without Borders here in Canada issued a plea to our government, based on their presence in Gaza, calling for ceasefire directly because of the unreasonably high number of collateral casualties.

At this juncture it doesn't seem likely that the numbers are precise, but even a much more conservative estimate would be quite shocking and aligns with action in the Syrian civil war (for example) much more than with action in western invasions or Ukraine. That suggests that, at the very least, insufficient effort is being put toward limitation of collateral casualties. It's also important to remember that while the three-to-one rule of casualties in war is very loose, we are likely looking at much higher numbers of wounded than killed. That is badly complicated by the blockades, lack of power and water, as well as current military operations against the remaining hospitals in the area.

Just my thoughts, not an expert.

From 1 look at their wikipedia page it seems so.

Both sides will say what is in their best interest. You see it in Ukraine war where both sides like to make up numbers, same with Hamas and IDF its in their best intrest to seem like the bigger victim.