There is only one (1) reaction that any character and creature can take: Opportunity attacks.
All other reactions are race/class/spell/item specific.
Remember – opportunity attacks occur when a creature leaves your melee reach. Which means if your weapon is a polearm, your reach is 10’. If your weapon is a short sword, your reach if 5’.
Some big bad baddies might have reach that is different than the standard. Keep that in mind as you maneuver around them. Remember – all creatures can take opportunity attacks.
The good news is that opportunity attacks are almost always melee attacks. Which means if you are fighting a spell caster, their melee attack is (usually) on the weaker side.