Breast feeding questions

First time mum and hoping some of you can answer some questions I have regarding breast feeding...

- If you don’t produce milk until 3/5 days after you give birth (as the nhs website states) then do you give your baby formula for them first few days then switch to breast milk?

- how do you know how much milk your baby is getting when they feed?

- I would like to express so OH can feed baby too but would i express into some bags and then freeze them? Or would they just go in the fridge? And then I would warm them up when ready to be used? And how long can they be stored for before needing to be thrown out?

- best breast pump out there at the minute that you can recommend?

- is it ok to mix feed? So sometimes breast milk and sometimes formula?

Genuinely have no idea & sorry if these are daft questions!