Multiclass spell slots are a bit more complicated than single class slots. You first need to determine the number of spell levels that each class grants you.
This is usually determined by your character’s level, but can also be determined by your class’s spell list. For example, a wizard has 4 spell levels and can cast at least 2 spells from each of their spell lists, so they would have 8 multiclass spell slots. .
Next, you need to determine how many spell levels each spell slot covers. For multiclass spells, this means that you must divide the total number of spell levels for your class by the number of spell slots you have.
This will give you the average level of a multiclass spell slot.
For instance, if a wizard has 4 spell levels and 8 multiclass spell slots, their average level is 1.5 (4 divided by 8).
Now that you know the average level of a multiclass spell slot, you can calculate how many total levels a multiclassed character can cast. To do this, add up the level of all the spells in your class list that are higher than the average level of a multiclass slot (this includes both single Class spells and multiclass spells).
You cannot cast any spells that are lower than this average level.
So, for our wizard example, if their class list has 3 spells at 1st level and 4 spells at 5th level, they can cast 3 + 4 = 7 Level 1 spells and 0 Level 5 spells.