[RPG] Wild Magic Surge – What does “You cast …” mean for your spell slots

In response to this question, I started to think about what it means when a Wild Magic Surge results in a "You cast spell-X".

So, "specific beats general" gives:

• You can cast a second spell when you would not normally be allowed to
• You can cast a spell you don't know
• If the surge forces you to cast a concentration spell you would lose any concentration spell that you currently had active
• Subject to what the specific entry says you can target it how you like
• Unless the specific entry prohibits it you could use metamagic on the spell

I am pretty confident in all of the above but if anyone disagrees please address in an answer.

Here comes the question.

How does this impact on your available spell slots?

"You cast …" to me suggests that if you have an applicable slot available then you have to use it to cast the spell because that is the general rule for casting spells but if you don't then "specific beats general" and it is cast anyway.

Specific examples:

You cast fireball as a 3rd-level spell centered on
yourself

This must be a 3rd level spell:

• if you have a 3rd level slot available you have to use it?
• if you don't, if you have 5 sorcery points and a bonus action available must you use these to get that slot?
• if you don't have any of that then the spell goes off anyway?

You cast confusion centered on yourself.

No level specified and confusion is a 4th level spell:

• if you have a 4th level slot available you have to use it?
• if you have a 4th, a 6th and an 8th level slot you can choose which one to use?
• if you have only a 5th level slot but also 6 sorcery points and a bonus action, you could choose to use the 5th level slot or use the points to create a 4th level slot?
• if you don't have any 4th level or higher slots but you have 6 sorcery points and a bonus action available must you use these to get that slot?
• if you don't have any of that then the spell goes off anyway?

And so on …