Okay, so that is in agreement with a statement I remember: The the common and uncommon Showcase Adventure cards (and their foils) can ONLY be gotten in the Collector's Boosters - therefore, inversely, the rares and mythics may show up in normal packs (just at a much lower rate, obviously).
In regular draft boosters (the ones you get from a regular box) contain:
Showcase card - Uncommon, Rare, and Mythic (Non-Foil) and all rarities (Foil)
Borderless Planeswalkers
The Extended Art are only on non-planeswalker, non-showcase rares and mythics and can only be opened from Collector's Boosters. There is one dedicated slot per pack for Extended Art, but then you may also get one in your Foil Rare/Mythic slot. Collector's boosters are also the only place you can get non-foil commons in one of the three Special Frame slots.
This means that the rarer cards in the sets will be the extended arts (which can also be found in foil) and the non-foil common showcases. Borderless Walkers can be found in both products.
"This is the booster you know, the one Magic has had for 26 years. The only change we're making to it is giving it a name. We're now referring to it as the Draft Booster. It's the booster intended for drafting. If you like Magic as-is and want to continue buying the same thing, you can. The card make-up for it is not changing (although, it will have the borderless planeswalkers and showcase cards)."
In regular draft boosters (the ones you get from a regular box) contain:
Showcase card - Uncommon, Rare, and Mythic (Non-Foil) and all rarities (Foil)
Borderless Planeswalkers
The Extended Art are only on non-planeswalker, non-showcase rares and mythics and can only be opened from Collector's Boosters. There is one dedicated slot per pack for Extended Art, but then you may also get one in your Foil Rare/Mythic slot. Collector's boosters are also the only place you can get non-foil commons in one of the three Special Frame slots.
This means that the rarer cards in the sets will be the extended arts (which can also be found in foil) and the non-foil common showcases. Borderless Walkers can be found in both products.
DracoBall: http://www.mtgsalvation.com/forums/the-game/casual-related-formats/homebrew-variant-formats/598529-dracoball-updated-3-24-15
Fantastic Fours: http://www.mtgsalvation.com/forums/the-game/casual-related-formats/homebrew-variant-formats/603605-fantastic-fours
From this article: https://magic.wizards.com/en/articles/archive/making-magic/project-booster-fun-2019-07-20
DracoBall: http://www.mtgsalvation.com/forums/the-game/casual-related-formats/homebrew-variant-formats/598529-dracoball-updated-3-24-15
Fantastic Fours: http://www.mtgsalvation.com/forums/the-game/casual-related-formats/homebrew-variant-formats/603605-fantastic-fours