D&D 5E (2024) Using Action Surge to cast spells in 2024

[Maxperson]

That seems to be the only distinction, 'cast spell' is used when it is an existing spell I can look up and 'use Magic action' is used when it is a unique effect. Of course a spell usually also requires the Magic action to cast, see the PHB

"Most spells require the Magic action to cast, but some spells require a Bonus Action, a Reaction, or 1 minute or more. A spell’s Casting Time entry specifies which of those is required." So any spell with a duration of action requires the Magic action to cast anyway.

That makes sense. It's their excuse for why taking the spell fireball from an NPC and instead giving it the Bloom of Fire(or whatever) that has the exact same numbers as fireball keeps it from being spellcasting.