What it is
Meta lets a Page run ads on Instagram even without its own Instagram account, through a Page-backed Instagram account. Nobody can log into it — it exists only for ads.
On launch
If the launch form has no Instagram picked, "As the Page only" isn't selected and the placements include Instagram, AdFleet picks the Instagram itself, in this order — whoever launches, buyers included: the Page's own Instagram → an Instagram connected to the Page → an existing Page-backed Instagram → the ad account's Instagram (only if it has exactly one; with several, the ad runs as the Page and the launch journal asks you to pick an Instagram in the launch form). If none is found, AdFleet creates a Page-backed Instagram with your BM's System User token — only on a Page this BM owns. For a client Page shared through partner access, or if creation fails, the ad runs as the Page.
The "Page Instagram" screen
The Owner and Head see in "Page Instagram" which Instagram each Page of your BM has. "Read from Meta" refreshes the list; for Pages without Instagram you can tick the ones you need and press "Create Instagram for the picked" — up to 25 Pages per run, but no more than the ad accounts in your plan; one by one, with the list shown for confirmation before anything is sent. Pages an agency shared with you aren't shown here.
What AdFleet doesn't do
It never creates or changes real Instagram accounts, never logs into them and never posts.