There currently exist both of the following questions:
- Can I cast a Warlock spell with another (higher) spell slot from a different multiclass?
- What level are spells cast at if a warlock multiclasses as another caster?
And there are two questions which are marked as duplicates of both of these:
- If I multiclass as a warlock and another class, and I get spell slots from that other class, do they count as my highest-level slot or not?
- Can a multiclassed warlock cast warlock spells on a lower level spellslot from their other class?
There are also four questions which are marked as duplicates only of the second one:
- Warlock and Sorcerer Multiclass Casting With An Enhanced Effect
- D&D 5e Warlock Multiclass - Casting Warlock Spells Using Other Class Spell Slots
- Warlocks using spell slots with multiclassing
- Paladin/Warlock sharing spell slots
This is just a request that the four directly above be marked as duplicates of both questions.