Tessarael
Explorer
Extra Attack grants additional attacks when you take the attack action. You can only make the attack action once on your turn. Hence, you can only gain the benefit of Extra Attack once on your turn.
On a critical hit, Devoted Assault allows you to spend exertion to use a Tempered Iron maneuver against the creature, providing that it can be activated with an action or bonus action. I'd interpret this as allowing you to activate either: (1) Striding Swings to move 15', including "through the space of hostile creatures that are up to one size category larger than you, and the spaces of other creatures do not count as difficult terrain"; (2) Stunning Assault to force the creature to make a Constitution saving throw or be stunned until the end of next turn, on the melee weapon attack that scored the critical hit and any remaining melee weapon attacks that you have left; or (3) Dispelling Assault to end a spell of 3rd level or lower on the creature on the critical hit, and on any remaining attacks that score a critical hit and the critical hit range for any remaining attacks is expanded to 19-20 as detailed in Dispelling Assault, or 17-20 if you have another feature that increases the range of your critical hits.
I.e., no additional attacks granted, but the benefits apply on the critical hit that you just scored, and on any remaining attacks per the wording in the Stunning Assault or Dispelling Assault maneuver.
On a critical hit, Devoted Assault allows you to spend exertion to use a Tempered Iron maneuver against the creature, providing that it can be activated with an action or bonus action. I'd interpret this as allowing you to activate either: (1) Striding Swings to move 15', including "through the space of hostile creatures that are up to one size category larger than you, and the spaces of other creatures do not count as difficult terrain"; (2) Stunning Assault to force the creature to make a Constitution saving throw or be stunned until the end of next turn, on the melee weapon attack that scored the critical hit and any remaining melee weapon attacks that you have left; or (3) Dispelling Assault to end a spell of 3rd level or lower on the creature on the critical hit, and on any remaining attacks that score a critical hit and the critical hit range for any remaining attacks is expanded to 19-20 as detailed in Dispelling Assault, or 17-20 if you have another feature that increases the range of your critical hits.
I.e., no additional attacks granted, but the benefits apply on the critical hit that you just scored, and on any remaining attacks per the wording in the Stunning Assault or Dispelling Assault maneuver.