Google's rules for asking
Ask every customer, in the same words. Google does not allow a business to discourage bad reviews or to ask only the customers it expects to be happy (Google Maps content policy).
Offer nothing for a review. Google says offering incentives, like free or discounted goods or services, in exchange for reviews counts as fake engagement and is strictly prohibited (Tips to get more reviews).
Leave the words to them. Google says a business should not ask for anything in particular to be written, or require or pressure customers to leave a review while they are still on the premises (Google Maps content policy).
They need a Google Account. A customer has to be signed in to a Google Account to leave a review, and Google says they can set one up with an email address that is not Gmail (Tips to get more reviews).
Google suggests sharing the link on receipts, in thank-you emails and at the end of a chat, and printing the QR code for your shop (Create a Google link or QR code to request reviews).