Re: Bathroom fan exhaust
If possible, I would try to tuck the top of the exhaust cover below the bottom edge of the siding so that it sits fully on one piece. This would allow caulking of the most important top and side areas. The bottom can also be caulked or install a small shim to fill the gap and then caulk both the shim to the wall and the cover to the shim.
If you can't do the install like that, then install however, then either use foam caulk backer or cut a wood shim and caulk everything to seal against critters and water intrusion.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!