How do Sainsbury's Nectar points work?

[Gingernaut]

I've got the Nectar app and Sainsbury's have restarted their offers.

One thing that confuses me is this;

If I'm offered, say 70 points for buying a bag of salad, do I get 70 points and then the offer expires after that one bag of salad?

Does the offer continue and 70 points are then added for every subsequent bag of salad?

Csn I only buy one bag of salad per shop?

Do I have to purchase multiple bags individually?

I'm confused.

During the lockdown, I don't want to shop multiple times a week, but wouldn't mind maximising my points.

An offer comes up and is time limited, ususally for a week. I expect you could buy as many as you like. If you have the App the offers come up and tell you the date they finish

Thanks