Some missions will be offerer offered to you directly but many can be found in, or become added to, the Mission Bulletin Board System (BBS).
The "campaignCampaign" (AKA "main plot") is only available through the campaign Campaign game start. All other missions are available in every game start. Although NOTE: if playing the campaignCampaign, some side plots (and DLC systems) cannot be accessed until the first gate (to DV) has been used, or after campaign the Campaign completes.
Links to info about the different types of missions: