When you use the Attack action with an Unarmed Strike or a monk weapon on Your Turn, you can make one Unarmed Strike as a Bonus Action
Immediately after you take the Attack action on Your Turn, you can spend 1 ki point to make two unarmed strikes as a Bonus Action.
Beginning at 5th Level, you can Attack twice, instead of once, whenever you take the Attack action on Your Turn.
Starting at 5th Level, you can interfere with the flow of ki in an opponent's body. When you hit another creature with a melee weapon Attack, you can spend 1 ki point to attempt a stunning strike.
You channel primal magic to cause your teeth or fingernails to sharpen, ready to deliver a corrosive attack. Make a melee spell attack against one creature within 5 feet of you.
Correct. Casting a spell, even if it includes a melee/ranged attack as part of it, is the Cast A Spell action. That is different from using the Attack action on your turn.So you are saying a melee spell attack is not an attack action?
Can a spell with an attack roll be used as the attack in the Attack action or as part of the Extra Attack feature?
The short answer is no.
As explained in the Player’s Handbook, you can take one action on your turn in combat, in addition to moving. You choose your action from the options available to everyone—options such as Attack, Cast a Spell, and Dash—or you choose from among the special actions you’ve gained from a class, a feat, or another source.
If you want to cast a spell on your turn, you take the Cast a Spell action. Doing so means you’re not taking the Attack action or any other action. It is true that a number of spells, such as fire bolt and ray of frost, involve making an attack, but you can’t make such an attack without first casting the spell that delivers it. In other words, just because something involves an attack doesn’t mean the Attack action is being used.
By extension, the Extra Attack feature (given by several classes, including the fighter and paladin) doesn’t let you cast extra attack spells. That feature specifically relies on the Attack action, not the Cast a Spell action or any other action.
In summary, to make a spell attack, you have to first cast a spell or use a feature that creates the spell’s effect. A game feature, such as Extra Attack, that lets you make an attack doesn’t let you cast a spell unless it says it does.