P – Potions – How prevalent is magic? Can one just go a buy it? Is it real magic or a charlatan?

In a high magic setting can one just go buy any magic item you want? In a low magic setting, with any “magic” items one can buy can be mere forgeries, or lies. Perhaps only with Nystul’s magic Aura on them? Or not magic at all? Granted, a high magic setting could have the same issue.

If a current low magic campaign, but the fallen city was high magic, what kinds of items will be found here, and where will they most likely be found, and how will the party discover them?

Will potions, scrolls, and spell books survive the ravages of time? Perhaps surviving books are one of the many librams and tomes suggested in the Dungeon Master’s Guide. Scrolls might be found it airtight and resilient scroll cases, or perhaps magical scroll cases to specifically protect scrolls. Potions could be in special bottles, or one might find a bad of holding or a portable hole with its original contents. Or such things found could be the remains of past adventurers seeking riches and glory.

Will a fallen city be fallen partly because of some dread magic item? Will this be an item they know the big bad is seeking, and seek to find it first?