Bards, clerics, druids, wizards, and sorcerers use full class levels on the MC chart. Paladins and Rangers use half class levels rounded down. Eldritch knights and arcane tricksters use a third of the class levels rounded down. Warlocks use a separate system and don't use any levels added to the MC spell slot chart, and other classes don't add any levels either.
For example, if you had a bard 6, ranger 4, arcane trickster 4, warlock 2 the character would be 16th level and any cantrips are at 16th level. Spells slots would be as a 9th level character on the MC chart -- 6 for the bard, 2 for the ranger, 1 for the rogue. There would be none for the warlock on that table, but the warlock would still have his own slots.
Any spell known can be used in any slot, and the related spell caster ability modifier is based on the class that learned the spell. Spells are learned separately. That character would have spells known from each class individually.
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