Check the stat block; it varies from creature to creature

Ogres have the standard 5-foot reach with their greatclub, and javelin if using it in melee, and can only attack melee targets within 5 feet. MM, p. 237.

Other creatures, typically larger than size Medium, may have melee attacks with greater than 5-foot reach, which is noted in their descriptions. (PHB, p. 195.)

For example: the Large-sized Pit Fiend has a mace with 10-foot reach. (MM, p. 77.)

The Aarakocra has a Str modifier of +0, and a Dex modifier of +2. As a CR 1/4 monster, it has a proficiency bonus of +2 (see the Proficiency Bonus by CR table, page 8 of the MM).

The Talon attack is the following:

Talon. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) slashing damage.

So based on the calculations behind player attacks, it seems likely that this is calculated as:

Dex (+2) + proficiency (+2) = +4 to attack

Dex (+2) = +2 damage

Obviously, if you try to use Str instead of Dex, this doesn't add up.

However, it's possible that creature attack bonuses are either arbitrary or not calculated the same way as player attack bonuses are, but if this is the case there is no way to answer this question.

Somewhat disconcertingly, the Aarakocra player race from the Elemental Evil Player's Companion gains:

Talons. You are proficient with your unarmed strikes, which deal 1d4 slashing damage on a hit.

Which would be based on Str, since the Unarmed Strike is not a finesse weapon.