I wouldn't make a habit of driving with the bike unprotected in the rain.
If the bottom bracket cable guide is located with a screw, you can undo that and let any remaining water out.
If you are in a damp climate, a spray inside a steel frame with frame saver or similar will help preserve the frame.
Alloy frames and other materials are less sensitive to corrosion.
And yes, all the frame tubing meets at the bottom bracket which has holes in it for them, so water will gather there. More so, waer thrown up by the rear tyre will channel down under the seat to the seat post, which if unsealed, will allow it to run down to the BB in the seat tube.
BB cartridge bearings generally are well protected from moisture and won't be affected, unlike old-style cup-and-cone jobs. All frames might run into problems with corrosion (rusting) or electrolytic reaction between dissimilar metals that will seize the threads.