Hello Syrinx,

during extart/exchange and loading phase unicast packets are used to build the OSPF adjacency

an OSPF adjacency is always between a pair of OSPF routers that have reached two way status.

typically a new router forms OSPF neighborships with all routers on subnet, but only two OSPF adjacencies with OSPF DR and OSPF BDR

for this reason communication becomes unicast in those phases: only two nodes are involved in the process.

Edit:

on the routers you can use

debug ip ospf adjacency

to see the process

Hope to help

Giuseppe