Regarding maximum wifi links in an area

Hi,

I am learning about communication and certain questions popped to my head...

i.Assume the same wifi provider gives many connection in an area. Does the frequency bands used be compulsorily be different(to avoid interference)?

ii.How many active links can be there? I've this question because, if many people are using the same frequency band, there can be a case when there is no free band of frequency.

iii.Do wifi modems or routers use some kind of orthogonal modulation or something, so that the signals from various sources donot interfere... on what does the number of sources in that case depene?

My community has municipal wifi. I can see a node with its antenna, sitting on a utility pole across the street from me.

My two computers each have a wireless detector. They can pick up the node's wifi broadcast. They detect it on one channel only, not several.

My computers also pick up other wifi networks within 100 feet or so. Those networks are on different channels.

I have a wifi bridge, and an ethernet hub-switch (Netgear brand). My computers are simultaneously able to communicate with the internet, to the same bridge, on the same channel.

With so many data packets going back and forth, networks have to cope with collisions. My ethernet hub-switch has an amber light labelled 'Collision'. When communication is especially active, the light flashes once or twice a second.

If the WiFi links are given by the same provider in a dedicated area, most probably all are on the same channel.
Latest WiFi systems (802.11 g,n,ac) use OFDM modulation which allows using the same channel by multiple users.

If they are all in the same channel, then what factors limit the number of :

i. Simultaneous active users
ii. Total speed a user can get

Please correct me if I'm wrong, but I feel that, whatever modulation scheme is used, a large number of users would cramp the the total bandwidth available to all.

Losing bandwidth and data speed always is a trade-off when allow multiple users using the same channel.
But some modulations doesn't allow at all using the same channel simultaneous by multiple users.