Hello everyone

Im slowly upgrading my Trek District S to my liking. I want/wish to keep it "lightweight & simple" (Ii'll post some pics later)
Can someone please help me shed some light on carbon fork specs. My confusion is where the steertube meets with the fork itself (where the bottom bearings sit). Does the diameter varies? it seems like almost all carbon forks I see are designed for integrated headsets. the headtube of my trek district is not that big so I need to know "How" can I buy a carbon fork online considering this specific fit on my bike. The one installed on the bike at the moment is a steel fork with VP slimstack headset (Im not 100% sure, I just researched online).

*The same late model of this bike uses a carbon fork (Bontrager Satelite Plus) if it helps. I dont know why Trek decided to down grade on their new models.

Thanks in advance