Magic Items
When I cast a spell from a magic item, how do I set the spell save DC?. Set the DC the same way you would when casting a spell without using a magic item (DC equals 8 plus your spellcasting ability modifier plus your Proficiency Bonus). If you don't have a spellcasting ability, your spellcasting ability modifier is +0 for the item, and your Proficiency Bonus still applies.
Can potions be administered to Unconscious characters?. Yes, you can administer a potion to someone else as a Bonus Action.
If a potion doesn't duplicate a spell, does it require Concentration?. A potion's effect requires Concentration only if its description says so or if it duplicates a spell that requires Concentration.
When should I use the rule for scrolls in the Dungeon Master's Guide, and when should I use the rule for Spell Scrolls?. The rule for scrolls is for scrolls in general, including a Scroll of Protection, and it allows you to try to activate a spell if you're literate. The rule for a Spell Scroll is specific to that type of scroll and introduces an additional requirement: the spell on the scroll must be on your class's spell list for you to read the scroll.
A Spell Scroll can be named in a variety of ways: Spell Scroll, Scroll of X (where X is the name of a spell), or Spell Scroll of X (where X, again, is the name of a spell). No matter how its name appears, a Spell Scroll follows the same rule.
For you to meet a Spell Scroll's requirement, the spell on the scroll needs to be on whatever spell list is used by your class. Here are two examples. If you're a Cleric, the spell must be on the Cleric spell list, and if you're a Fighter with the Eldritch Knight subclass, the spell must be on the Wizard spell list, because that is the spell list used by your subclass.
Do the AC bonuses from a Ring of Protection and Bracers of Defense stack?. Yes. In general, bonuses stack unless the text explicitly says otherwise (such as effects from the same spell). For instance, you can't benefit from more than one Ring of Protection, because the text says you can't attune to more than one copy of an item at a time.
My Fighter attacks a creature with a magic Longbow and nonmagical arrows. Is the attack magical?. The attacks made by a magic Ranged weapon are magical even if the ammunition is nonmagical.
Are magic weapons with the Thrown property (Javelins and the like) assumed to return to their owner after being thrown?. If a magic weapon returns to you after you throw it, its description says so, as in the Dwarven Thrower.
Can a Sphere of Annihilation enter a Leomund's Tiny Hut?. Yes. The sphere destroys the dome.
Can you gain the magical bonus of a +2 Shield if you are holding the Shield without taking an action to don it?. No. You must don a Shield to gain its benefits.