I have a question about backfill and indexOnWrite cloud functions. Could I edit them? I want to send timestamp milisecond to typesense instead of seconds for now

To "edit" it, you'll essentially have to copy the source code for that function into your own custom cloud function, and then disable the Firebase extension

where can I copy that source @Jason Bosco?

From here: https://github.com/typesense/firestore-typesense-search/tree/master/functions/src

thanks bro