Shall I fill those gap in the loft?

[imiami]

Please see the pictures, can I fill it with insulation or it is for air vent?

Air ventilation, leave them open.

Thanks, it is super drafty, i think I should cover 99 percent and leave a bit gap for air

No don’t do that, you will have a serious mould and damp problem if you do. There are cold roofs and warm roof systems. If yours is a cold roof you need to leave it alone.