Venice to Interlaken on Berner Oberland Pass

Hi,
I have purchased the Berner Oberland Pass. I am currently in Venice and plans to travel to Interlaken. I understand a connecting train is required but I am not sure which part of the following routes will the Berner Oberland pass kicks in.

Venice st.Lucia -> Milan Centrale -> Spiez -> Interlaken (Does the BO pass kicks in from Milao Centrale onwards or only from Spiez onwards?)

Venice st.Lucia -> Milan Centrale -> Domodossola -> Brig -> Spiez -> Interlaken (When does the BO pass kicks in?)

I will go with the option that BO pass kicks in earliest so as to maximise the usage of the BO pass. But the travelling tiem should not be too lengthy.

Appreciate your kind advice! :-)

Rgds