[Dusk]

How do Permanent spells work in 5e?

1. Do their effects only come into play after the spell has been made permanent (ie: sustained for Force combat turns)? Or..

2. Do their effects come into play immediately but the effects don't become permanent until the spell has been sustained for  Force combat turns?

Eg: Someone casts a F8 Mana Static (SG 117).

1. Does the BGC of 8 only come into play after 8 combat turns of sustaining has passed?

2. Or..Does a Background count of 8 manifest immediately, but does not become permanent till it has been sustained for 8 turns (if the spell is stopped sustaining in the meantime. the BGC of 8 goes away instantly)

[Reaver]

CRB page 283

Duration: This describes how long the spell’s effect
lasts. Instantaneous (I) spells have an effect that
happens and then fades, while Sustained (S) spells last
as long as you keep them going. Permanent (P) spells
don’t fade or dissipate; their effect becomes a lasting,
non-magical characteristic after you sustain the spell for
(Force) Combat Turns.

basically option 2.

You cast the spell (in your case force 8 ), and the effect begins. HOWEVER the effect is not permanent until you have sustained the spell for 8 combat turns..