Reprojection: UTM/WGS84 -> Geographic Lat/Lon

Hello, I am new to map projections and would like to understand what happens when I reproject S-2 L1C product (UTM/WGS84) to “Geographic Lat/Lon (WSG 84)” to be used in Google Earth.
Can I say that the original projection is “flat” map projection and the resulting image is in spherical, geographical projection?
What confuses me is that the original projection is also in lat/lon coordinates. So how does the projection actually change when going from the original (UTM/WGS84) to Geographic Lat/Lon (WGS 84) ?
Thank you.

Map projections are nothing to understand after reading a few sentences.
But here are a few links I find helpful.

But to make it short. UTM coordinates are not given in lat/lon but in meter. It’s the distance to a reference point. But they can be translated into lat/lon. That’s what we do in SNAP. We show lat/lon as default. But you can also have a look at the map coordinates. Open the Pixel Info View to do so.