This is anectodal as it just relates to the small scale organic grocery store I was recently rehired at after finishing university, so keep that in mind.
I last worked at this store very shortly before the pandemic broke out. While I was working there the margins were roughly industry standard with around a 30% markup for regular grocery items. Some hot sellers had bigger margins, other items had less. My position lets me look in our system to see the markup on certain goods. Now, after being absent, most items I see average about 40% markup with some items even getting up to 50% markup. This means you're paying half of the cost of the item to the store for the honor of having them order it and place it on the shelf for you. Even meat, which used to have a margin of about 20%, now roughly sits at 40%.
I was honestly shocked to see this change. For those who are unaware, this difference in profit is completely taken up by the store itself, not anyone else in the supply chain.
While this is a single case and any trends from what I have noticed shouldn't be extrapolated too far, I do personally think the same thing is happening all across different grocery chains. At least in Canada, where I live. The price of groceries here has skyrocketed since inflation began. Loblaws, a Canadian grocery monopoly, has had record profits since the pandemic and inflation. With this in mind, given that the price increases I've seen at the store I work at are roughly the same as the larger chains including Loblaws, I'm inclined to believe they have had a similar increase in their margins as my store has.
In the mythology of capitalism we are told that competition brings down prices. This seems like an obvious case where that is not happening.
Oh, and to kick it all off, I'm making 50 cents less an hour than when I last worked here.