There seems to be a lot of confusion here.
The universe is flat, or almost flat. Internal measurements of our universe are consistent with a flat universe. And, of course, the amount of possible error in the measurements means it could be very slightly curved.
The observable universe is limited by the distance where its expansion is equal to the speed of light. We can't see anything beyond that because any light from it would never reach us. The observable universe is estimated at 46 B-ly in diameter.
A brief history of the universe
The universe started when gravity precipitated out of the other forces creating time, space, mass, and energy. Before that, nobody knows what existed since there is no evidence from there. It could be an empty void, or it could be an infinitely dense energy field. Nobody knows.
At about 10⁻³⁰ seconds after the Big Bang, the other forces, that is, the Strong Nuclear force, the Weak Nuclear force, and the electromagnetic force precipitated out from each other. This cause to universe to hyper-expand, increasing its size by a factor or 10⁷⁸ in 10⁻⁵⁰ of a second. The resulting size is estimated to be several thousand light-years in diameter.
Between that time and the CMB (cosmic microwave background), the universe was full of electrons, protons, and photons. There were other quanta presents but they were a very tiny fraction of the whole. The universe continued to expand during this time.
The CMB happened when the expanding universe got cool enough for the electrons to drop into orbitals of the protons, creating hydrogen atoms and allowing the left-over photons to freely travel space. Its these photons that we measure as the CMB.
From then on, the universe continued to expand and gravity pulled the hydrogen together, eventually forming stars, galaxies, planets, and everything else we see today.