Right, lots to unpack here.
Firstly, Openreach, Virgin, Hyperoptic and City Fibre are all different network operators that have their own physical infrastructure. The only crossover between them is likely to be the use of Openreach PIA that allows sharing of ducts, at cost to the operators.
GPON and XGS-PON are different generations of passive optical networks. GPON has a total capacity of 2.4Gbps down and 1.2Gbps upload per port on an OLT. This capacity is shared between all the customers who are passively split on this port. The split ratio isn’t always public knowledge but tends to be something like 1:32. Openreach will have profiled usage on their network and arrived at a ratio that allows them to sell a range of speed packages without compromising service. The reason uploads are so much lower is because there is simply less overall capacity. XGS-PON is effectively the same but the total capacity is a symmetrical 10Gbps.
Because both technologies occupy different parts of the electromagnetic spectrum, they can coexist on a single fibre. This means that over time, operators like Openreach can introduce XGS PON without impacting existing GPON services. Equipment on the consumer end will require changing to access the new technology but this can be done as and when customers elect to subscribe to an XGS PON package.