How does minimum ilvl work?

[Jojoya]

Later today, a couple of friends and I want to tackle o11s and some old dungeons at minimum ilvl but keep our level 90 skills.

For example The minimum ilvl for o11s is ilvl 380 and we want to go into the raid at ilvl380 with our level 90 abilities. Would the minimum ilvl option sync our ilvl and class level? or only our ilvl?

If you want to keep your level 90 toolkit while doing the content at minimum ilvl, you would need to equip gear at that minimum ilvl (or gear that would average to that ilvl) instead of using the Minimum ilvl setting in Duty Finder. As EorzeaWanderer said, using the Minimum ilvl option in the Duty Finder would also sync you to level 70 and abilities available at level 70.

Something to keep in mind is that when your gear is ilvl synced in content, you lose the stat increases gained through any materia on your gear as materia doesn't scale in synced content. For dungeons, losing those stats shouldn't make much difference but it could be noticeable in something like O11S depending on how much extra power you gain through having access to the level 90 toolkit. You may want to go in with the ilvl 380 gear unmelded at first but then meld it if you're having problems meeting DPS checks.