I have been a secret shopper or mystery shopper for many types of businesses, including Walmart, movie theaters, and a variety of chain restaurants. There are tons of articles out there about being a secret shopper but I'm going to share my actual, personal experience being a shopper.
The first thing to know is if you are looking for work like this, the pay is very low, can take many hours to complete, and requires exacting standards. This is not a way to get rich and isn't even a great side hustle. Many shops either only pay you in reimbursement of your meal or perhaps a $5-$10 bonus if you complete the shop correctly. Secret shopping is best for people who have a LOT of free time, are very good at following specific rules/lists about what to do during a shop, who enjoy the work and don't mind spending a couple of hours doing a shop even if they don't get paid much.
It's also important to note that most mystery shops in restaurants require that you do NOT take your phone out at all during your visit to check the list of steps or have anything on your person that might show a server that you are doing a secret shop. So you have to conduct everything on the downlow and use the timer on your phone under the table. However, if you want to get a free meal once in a while, this is a way to do that.
Non-restaurant shops: There are a ton of variety with these. It can be as simple as calling a business to ask a list of specific questions and then following up with an in-person visit to see if they answer the questions the way they are supposed to. These can be short and fairly easy. There can be some great non-restaurant shops if you have flexibility in your schedule. For example, a couple of times I have gotten a free night's stay at a local hotel simply by doing a secret shop and noting whatever the list of things is they've asked me to check for the assignment, such as if everything is in working order inside the room, what the breakfast is like, if public spaces like a pool or gym are clean and in good working order, what the situation is with free coffee or if you ask about certain attractions. You can usually bring up to 1 person with you though typically not a child, so it can be a nice change of pace to stay somewhere else for the night.
Some of the non-restaurant shops are very cumbersome and can take several hours. For example, once I did a Walmart shop that required checking their entire non-alcoholic beverage section and taking pictures of every single section in each aisle and endcap that had beverages. Walmart has a LOT of beverages so that ended up being close to 200 pictures. I have a bad back so I can't stand for super long periods of time so I had to keep sitting down after taking some of the pictures as my back was very bad that day, so it took me like three hours to complete and when I was all done and uploaded all the pictures my "reward" was a whopping $30. But hey, when you have no income, $30 is $30.
I have also done shops and audit visits to movie theaters, where I've had to do things like check that they have stand-ups posted the right way and visible to guests, that there are flyers where they are supposed to be, to sit in the theater and count how many ads there are before the movie plays and note what ads they are, to take a general headcount in a theater, or do to things like present a letter to the manager and be let into a room to count 3D glasses as an audit. These are pretty easy but can be time consuming as the theater isn't always near where you live, and again, the pay is pretty low.
Restaurant secret shops can be harder to get. There are some chains that use specific companies and they can be tougher to get into. But some of the places I've secret shopped for food include gas stations and low-level chains like Charley's and Bob Evans. With sit-down restaurants that get shopped, servers have specific "steps of service" they're required to follow and you have to check whether or not they are following each one. For example, you may need to time how long it takes for someone to speak to your table and make a note of it on your phone. Then how long it takes from placing a drink order to receiving that order, and so forth throughout the whole meal, including how long it takes for them to clear dirty plates and give you the check, cash you out, etc. along with what they say during each step of service. Typically you also need to visit the bathroom once to make sure it's clean and stocked and you can take some notes on your phone while you're in there, but you don't want to be gone too long or you will miss some of the service steps.
Most typically, the shop will be for you and one other adult (optional), and the company will only reimburse you up to so much money. They may require you to order specific foods or beverages so it's best not to go with someone who is picky. You usually get reimbursement fairly quickly, and sometimes there is a $5, $10, or even a $20 bonus you get for completing the shop, especially if it's one that really needs to be picked up and nobody has selected it.
If you have the time, you can complete 2-3 shops in a single day and maybe make $50. Like I said, not enough to live off of, but if you hustle and don't mind talking to strangers, secret shopping may be for you.