(adding just to flesh out details alluded to in other answers)
QGIS uses a coordinate system for any map you show (canvas) or print (layout). Based on your settings under
Settings / Options / CRS, this may default to a projected CRS you specify, the CRS of the first layer you add, or -- as a last resort -- the unprojected CRS
EPSG:4326 - WGS 84, which is just x=longitude and y=latitude. So if you don't change anything, and don't (re)project any layers, you will end up with this implied "choice", whatever it may be.
All projections create distortions. The "Geographic", i.e. (non)-projection EPSG:4326, in particular, when used as (x,y) coordinates for a visual map, tremendously elongates apparent distances E-W far away from the equator. So it's unlikely to be a good choice for anything other than a very small-scale map right at the equator if you care at all about whether it "looks like what you experience on the ground". You should choose something better as the project CRS and/or the CRS for the map on your layout.
A word on terminology that may be causing confusion here. QGIS cheerfully reprojects layers on the fly, so (as others have said) you don't need to create a copy of a layer reprojected to your desired (project) CRS just to work on a good map, or even to do basic geospatial analysis. However, projections impact measurement (length, area, scale...) and projection mismatches can get in the way of processing involving multiple layers, especially if you use algorithms from other platforms that may be imperfectly integrated. So I would manually reproject if you're planning to work with a layer in a meaningful way (vector or raster).