Bikes can be kept outside without harm if properly prepared. For say a steel bike - framesave it, have it painted with say Imron or epoxy (brush painted epoxy works very well), lube bearings with lots of marine grease, chain with a durable wet lube, all threads with marine grease including spoke nipples). Grease cables with Phil Wood or the like including the several inches going into and out of lined housings.

Leather seats will be a challenge. I'd go with treating it with SnoSeal oe equivalent and perhaps covering it with dry cloth, then a plastic bag over. Plastic will not like sunlight. How poorly it will do depends in large part on the type of plastic. I'd be more comfortable with a bike with no plastic anywhere it mattered). Many handlebar tapes will fare poorly or fade.

I don't keep my city bikes outdoors but I expect them to sit perhaps many hours outdoors in Portland weather, be ridden home and put away wet. (All my bides get put away wet or left leaning against the workbench until they dry. All my bikes get hose rinses after dirty rides except in the dead of winter when the hoses are shut down. I expect to need to lube the good bikes before the next ride. City bike chains get FinishLine MTB wet lube.

Bikes are just like everything else. Do the proper prep and sitting outside isn't a big issue for 95% of it. Exceptions are things like plastic, bar tape, the paint color - all affected by sunlight and UV. Local weather needs to be understood. Air moisture and cycles of heating cooling and condensation. Perhaps freezing.

A lot of this is easier when starting from scratch with a bike where looks aren't everything. I picked up a sport Schwinn frame that had been in a crash. Straightened the frame, stripped the paint and brush painted it with 2-part epoxy. Built it up with parts off my previous all weather bike. That Schwinn got parked all day locked to a lamp pole in Ann Arbor most winter days (and ridden in Ann Arbor salt).

I've never owned a CF bike so I haven't spent much time thinking about what that would need/want/require. UV and sunlight might be a large issue. Electronics? Again, I've kept my bikes simple so no advice here. Theft obviously needs to be thought about and different areas are very different in that regard. Tires are not al equal. And there are plenty of other very specific issues that can pop up. I had water freeze once inside the front wheel rim, making the wheel so unbalanced it was almost unride-able. (Cycle Oregon. It had rained for 4 days. No way to dry anything. Bike outside the entire time. (All possible shelter was grabbed up fast by the earliest of the 2000 riders,) Last of those nights the temp went into the 20sF. That next day was 80 miles and I wanted to arrive in camp relatively early. Had no idea what was happening until I put the bike on the pavement and tried to ride it! Rode gingerly and away from all other riders until the sun finally came out and melted the ice.)