dnd 5e – How does Magic Stone interact with the Two-Birds Sling?

D&D 5e’s Mythic Odysseys of Theros includes a magic item called the two-birds sling. The item provides a +1 to attack and damage rolls made with it, and further provides:

When you make a ranged attack with this sling and hit a target, you can cause the ammunition to ricochet toward a second target within 10 feet of the first, and then make a ranged attack against the second target.

Meanwhile, the cantrip magic stone imbues pebbles with magic that allows one to

make a ranged spell attack with one of the pebbles by throwing it or hurling it with a sling.

This ranged spell attack “adds your spellcasting ability modifier, not the attacker’s,” to the attack and damage rolls, and on top of that the pebble deals better damage than a normal sling bullet (i.e., d6 versus d4).

How do these game mechanics interact? For example, imagine that Drew Druid casts magic stone and hands three magic pebbles to his ally Filippa Fighter. Filippa, being an 11th-level fighter with the Extra Attack feature, has three attacks to make on her turn. She uses her two-birds sling to hurl each of the pebbles as a ranged spell attack. Each attack hits, and as it does, Filippa gets to make another attack. But what kind? The sling‘s description merely says “a ranged attack” — which conceivably could be a ranged spell attack (because Filippa used a magic stone pebble) or a ranged weapon attack (because she used the sling). And what is the damage dealt by these additional attacks if they hit?