Aircraft are a different animal, so let's start with campaigns and missions. First of all, many will come with the proper directory structure within their zip file, so that you can unzip the files in your BAT root directory (the directory where the il2fb.exe lives). All campaigns and missions should come with a readme that will tell you how to install...something like this:
Extract the TempestV.zip file to a temporary directory.
• Copy the decompressed Missions folder to your IL-2 1946 root folder.
• In the New Pilot Career page, choose RAF in the Air Force selection drop-down. Choose Tempest V in the Career drop-down.
• We recommend turning off No Instant Success for this campaign.
• The campaign is designed to be compatible with any rank selection.
Sometimes the zip will contain other files, such as skins. Same deal; see the readme for the instructions.
Generally, all campaigns and single missions to into the Missions directory. Campaigns will go into Missions/Campaign/XX where XX is the country code of the campaign. So an RAF campaign goes in Missions/Campaign/GB ("GB" for Great Britain).
Similar process goes for Single missions. The directory is Missions/Single/XX.