The Brotherhood of Steel originally developed in California, with a second contingent based in Appalachia (West Virginia). This second group was in contact with the first until communication systems broke down, and the Appalachia group was wiped out not long after this.
The Brotherhood later expanded from their California location, with well-known chapters located around Vegas, Washington DC, and Boston. There are additional references to locations such as Montana, with non-canon locations in Texas, Chicago, and Colorado. Other canon and non-canon groups are referenced vaguely at times (usually without naming locations); some are splinter groups, while others are expeditions that don't permanently settle in one spot or are wiped out.
In the non-canon Chicago chapter, they expand and grow to control a large Midwest area, but are eventually reduced to a small group after a number of wars and further problems. They have contact with other Brotherhood groups, but the details of those contacts are not well known, beyond that the main group and the Chicago group do not see eye to eye.
In canon, there is still a Chicago group that arrived by airship and were forced to fight against super mutants. Conversations in Fallout 4 are unclear about the status of this group; they are known to have existed and are considered rogue members of the order, which necessitates some form of contact taking place. But other mentions of them seem to indicate this group was wiped out. See footnotes 6-9 here for the direct quotes.
To my knowledge, no real interaction between Chicago and any other BoS group has been well documented/discussed/shown in any Fallout media, canon or otherwise, other than obscure or partial references. We know they had interactions of some sort, both in canon and not-canon, but the details are left to speculation.
