D&D 5e Multiclassing Spell Slot Calculator
Restore 2 spell slots of up to 3rd level or 1 spell slot of up to 5th level.
How does a damage dealing Wizard use higher level spell ...
Within the Levels is where the Spell Slots (charges) are stored. Using Shadowheart as an example, early on she can have four charges in Spell Slot Level 1 and two charges in Spell Slot Level 2.
Artificer Multiclassing and Spell slots - forums.giantitp.com
Some spells require Spell Slots of a certain level, like the 2nd level Scorching Ray needing a 2nd Level Spell Slot or more to work, and not being able to be cast with a 1st Level Spell Slot.