The Fey Touched and Shadow Touched feats published in Tasha's Cauldron of Everything each grant the use of two spells. Among other things, they say:
- You learn the [spells].
- You can cast each of these spells without expending a spell slot [once per long rest].
- You can also cast these spells using spell slots you have of the appropriate level.
The spellcasting ability for these spells is specific to the feat, so it may or may not be the same as the ability of a class that grants spell slots.
Casting classes all have their own quirks regulating their spellcasting features, but these shouldn’t be relevant to how the Touched feats’ spells are cast in #2 because that casting isn’t using a spellcasting feature. This is clear because even non-casting classes can cast the spells in this way, so only the generic spellcasting rules apply there.
(This is related to why racial feat spells can’t be cast with a focus, as discussed at If a spellcaster's racial trait grants a spell that requires material components, can they use their class' focus to cast that spell?)
However, things get more complicated when the feats’ spells are cast as described in #3, because the spell slots referenced there do come from a class spellcasting feature. In that case, does the caster simply use the spell slot as “fuel” and otherwise cast the spell exactly as it had been cast for #2? Or is this inherently different, with the caster actually using their class spellcasting feature and therefore having to follow all of its specific rules?
Furthermore, is the answer different depending on whether the specific Touched feat spell being cast is on the caster’s class’ spell list or not?