An unnamed source from Larian answered WCCFTech, "When multiclassing, the levels of your classes and subclasses that normally have access to spell slots are added together in a weighted formula, and then the overall spellcasting level is used to determine how many and which levels of spell slots you will receive. Warlocks are an exception here in that they only get their fixed-level spell slots."
Maybe this is the "weighted formula": "You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table."
