What the block does
It renders the reviews attached to your shop as part of the page, in the order and position you put it. Like every other block, it is optional. A new shop with two orders is better off without it, because an empty or thin reviews section draws attention to how new the shop is.
Ask at the right moment
The moment is after delivery, not after payment. A message in the same chat a day or two after the parcel arrived gets answers, because the buyer is holding the thing you sold them and the conversation is already open.
Ask for one specific thing rather than a rating. What did you use it for. What surprised you. Those answers are readable and they are the ones future buyers act on.
Do not manufacture them
Invented reviews are obvious to the reader and they poison everything else on the page. The same goes for pushing a rating up by asking only the customers you know are happy. A shop with four honest reviews and one lukewarm one reads as true, which is the entire point of having them.
Answer the critical ones in public
A calm reply to a fair complaint sells more than the complaint costs. It is the only place a buyer can see how you behave when something goes wrong, and that is what they are actually trying to work out.
Where to put it
Below the product grid, above the FAQ. A buyer who has already seen what you sell is ready for what other people thought. One who has not is being asked to care about strangers before they know what the shop is.