Can warlock spell slots be used to cast above warlock casting level when multiclassing? [duplicate]

One of my players is considering multiclassing warlock/cleric. I have not played warlocks or had other players use them so I am looking into the casting mechanics. After looking at other threads I think I have the idea down. Something like a warlock 1/cleric 5 would have the cantrips and 1 warlock slot that can cast at first level and the cleric would provide up to 3rd level spell slots. my confusion comes from

Pact Magic: If you have both the spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the spellcasting class feature, and you can use the spell slots you gain from the spellcasting class feature to cast warlock spells you know.

so they would be able to cast warlock spells up to 3rd level with the cleric 3rd level slot, as evident from answers to this question, but would they also be able to cast fireball (in the case of a light domain cleric) using their warlock slot but doing so at 1st level? If so how would this translate to an end game build? would the cleric then have the equivalent to 2 9th level slots?

I am assuming this does not work this way and the initial build listed essentially just has a slightly expanded spell list and an extra 1st level slot that comes back on a short rest, but I just want to make sure I am not missing something so I can explain this fully in the future.