Depending on your BTS type, there are different configurations, e.g. star, multi-drop and loop configurations.
You can go from the BSC to the first BTS, then from that BTS to the next BTS, and from that BTS yet again to another BTS. This topology might be cheaper since you dont need as many E1 line cards at the BSC, and the lenght of the E1 lines might be shorter between two BTS rather than going all the way to the BSC from every BTS.
Regarding ‘how it is formed’: It’s the same like on any ISDN link (Q.921). Each BTS has one OML link which is established using a fixed TEI establishment procedure. Every TRX has one more link, each with their own TEI.