did the US, just conquer a whole nation overnight?

if you only listen to US media you might think so; at the press conference today Trump said "we'll run it" as if they already control it, and he also claimed that Vice President (now acting president) Delcy Rodríguez told US Secretary of State Marco Rubio that "we'll do whatever you need". However, a few hours later:

The Vice President of Venezuela demanded the release of Nicolás Maduro and Cilia Flores during the National Defense Council, reaffirming Maduro as the sole president and at the same time announcing the activation of the Citizen Security Agency and national power to defend independence, sovereignty and territorial integrity, which they denounce as attacked.

so, no, although the US has kidnapped their president, at this point in time they have not actually conquered the nation.