From the campaign creation flow, in the Audience Step, you should see the following placements, with some already selected for you:
Apart from the straightforward Mobile/Desktop limitation that is up there, let's look into the different platforms you can display your ad on.
The first three options will usually be auto selected, but it depends on the Campaign Objective you'll be using.
To get access to Instream video placements for instance, you'll need to use the Brand Awareness, Video Views or Post Engagement objectives.
For Instagram, both the Stream and Story placements will be selected as Default.
Pay attention to the format of the ad you're using, as not all of them fit the Story placements!
Messenger's Sponsored Messages format is exclusive: you can't use it with any other placements.
Finally, the Rewarded video is limited to the Mobile App Install objective.
Finally, you'll see the Advanced mobile placement option, which show this when expanded:
From here, you can focus only on specific Operating systems, or limit your audience to users connecting from either Wifi or a Mobile Connection (3/4G)
Any of placements you will choose will appear in the recap screen on the right: