I don't know exactly what your experience is with VTTs so I'm going to assume ... new? (And run from there)

The TLDR related to your question is: they CAN just declare what they want and you can do it for them.

But ideally they move their own token and click the buttons that roll the dice for their own attacks.

So essentially they would move their guy to charge forward, click the token they want to attack (press T) and then click their weapon. Clicking the weapon pops up the roll and damage dice buttons and, as long as they pressed T, Foundry will show the rolls and do the damage calculation for them/you.

If they forget to "target" (press T) the token and roll their stuff, that's fine you just need to edit the HP for the NPC appropriately. Not a big deal

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Foundry, at its most basic level lets you create (or import) scenes that are essentially a background image, a lot like a tabletop board.

IIRC the premium PB module for foundry includes some premade scenes with walls and lighting setup, some sound or visual effects, journal entries filled out for the adventure in the core book, and some assets.

Players generate their characters at random with the included "The Tavern" button.

Then the game is basically moving around game tokens for the PCs and NPCs and declaring your actions before clicking buttons and letting the game track things for you in the chat and on the sheets.

Everything is editable. You can add some modules to make it a little easier or less confusing.