What do high level fantasy cities look like? How are they different from other cities
You can go home now, folks.
Are there suburbs of sigil? Like certain doors which consistently open to the same place, so mid level wizards take up residence there to avoid the commute and also being horribly murdered?
No, the only way in or out of the city is through portals. And it's a rather dangerous places since there are portals to pretty much every plane of existence. So angels and devils walking the streets, factions vying for power and just about anything else you can imagine.