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Aura of Protection on PHB p. 85 states:

Starting at 6th level, whenever you or a friendly creature within 10
feet of you must make a saving throw, the creature gains a bonus to
the saving throw equal to your Charisma modifier (with a minimum bonus
of +1). You must be conscious to grant this bonus.

(Emphasis mine)

This makes me think only one creature gains the benefit, either you or a single ally in range, because:

• It says "or" instead of "and"


• It says "a friendly creature" instead of "any friendly creatures"


• It says "the creature gains" no "the creatures gain"

For example, Aura of Courage states:

Starting at 10th level, you and friendly creatures within 10 feet of
you can't be frightened while you are conscious.

The benefit applies to all allies within range, including you

The key word that makes this apply to multiple creatures is "whenever". This word means that the ability applies the benefit every time the condition is met. Hence, all saving throws made by you or your allies within 10 feet of you while you are conscious get the bonus.

There are other similarly worded abilities, such as the Devotion paladin's Holy Nimbus (all emphases added):

Whenever an enemy creature starts its turn in the bright light, the creature takes 10 radiant damage.

or the Oathbreaker paladin's Dread Lord aura:

Whenever an enemy that is frightened by the paladin starts its turn in the aura, it takes 4d10 psychic damage.

All of these abilities are tied to auras, and having an aura that only applies to a single creature within range doesn't make logical sense (especially when other paladin auras unambiguously apply to everything within range).

On the other hand, as you say, there are other abilities that could be worded in this way but are not, such as your example (Aura of Courage) or the Oathbreaker's Aura of Hate:

Starting at 7th level, the paladin, as well any fiends and undead within 10 feet of the paladin, gains a bonus to melee weapon damage rolls equal to the paladin’s Charisma modifier (minimum of +1).

This could have been written something like "whenever the paladin or a fiend or undead within 10 feet deals damage with a melee weapon, the creature gains...". Regardless, the two wording structures are functionally identical: all of the above abilities, including the ones quoted in your question, apply their benefits every time the conditions are met. These abilities are all written in plain English, and there are always many ways to say the same thing in plain English. It seems that the writers of these rules have used whatever wording feels most natural for each ability individually, rather than establishing a "standard" wording ahead of time and then using it whenever a spell or ability applies to any creature satisfying certain conditions.

If you had to choose a creature, the ability would tell you

Generally, the rules explicitly spell out any choice that must be made by the player. For example, another part of the Oathbreaker's Dread Lord aura says:

Additionally, the paladin and creatures he or she chooses in the aura are draped in deeper shadow.