Differences of course, generally similar. A collection of nations vs a collection of worlds.
There is a provision in it's charter for the UN to have it's own military, but the member nations won't allow it.
Really?
Got a link?
https://www.un.org/en/sections/un-charter/chapter-vii/ One could interpret this chapter as granting the UN powers to create a standing army.
Apparently the idea was floated during the Joe Biden back in the 90s: https://www.heritage.org/node/21052/print-display
What do you see as the federation changing from what it was during TOS, to something different during later series?
The Federation feels more separate in the TOS era. The colonies are not referred to as Federation colonies. They are Earth colonies. The Enterprise operates under the authority of the United Earth Space Probe Agency. A Starfleet Captain and Chief Medical Officer lacked knowledge about some key Vulcan biology and customs such as an inner eyelid, Pon Farr, and koon-ut-kal-if-fee. Vulcan seems much more alien and mysterious. This would seem quite odd if it was applied to a cohesive union like the United States. Imagine someone from Massachusetts not knowing some pretty big points of biology or culture about people from Wyoming. The Federation in TOS feels like a UN in the sense of a loose collection of nations and not a tight, cohesive union.
In contrast, the Federation from the TNG era and beyond feels much more like a unified whole. They no longer refer to Earth colonies like they are separate than other Federation colonies. Federation rights seem much more unified and we don't see (to my knowledge) a planet like Stratos where they have a caste system and another planet like Earth or Vulcan where such a system would be illegal.
In TOS the Enterprise was much further away from contact with the government and communications were slower. By TNG it seemed like the Enterprise was always a comm call away from Federation authority and could get instant replies.