From the campaign creation flow, in the Audience Step, you should have the following options to select your placement:
Selecting Auto means that you will let FB decide where to display your ad based on the performance. If you decide to manually select the placements you'll have the following options to choose:
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).
You can also Apply a block list of apps or websites within Audience Network where you don't want your ad to be displayed or Exclude a complete category of websites and apps. You can read more about Block Lists here.
Any of placements you will choose will appear in the recap screen on the right: