There are two ways to do POIs. One off - just simply place what used to be a label on the screen. It is still usable as a label, but if you attach a demand curve to it, it will function as POI. You can place as many as you want.

The second way is to create a mod POI. There are quite a few available in the workshop, download and take a look at their structure, which is also what is in the mod development guide (there is a section in there). People use POIs for places like airports, stadiums, shopping malls, remote cities where you don't want to develop city transit structure and instead have everyone "live" close to the train station.

A rule of thumb is with 100% global demand and "normal" demand curve, you should expect about 20-25% of the listed POI population to board transit over 24 hours. This is because there is a 50% adjustment coefficient built into the demand calcs, and because standard demand curve over 24 hours averages to 40-50% of the maximum (presuming peak hours are at 100%, night at close to 0%, and in-between at some average values 50-60%). You may need to adjust the POI numbers based on the demand profile. For example, a stadium may only draw people 3 hours a day once a week. So to get 10,000 people all take transit to the stadium, you may need to set the POI at a high multiple of this (e.g. 100,000). Do trial and error to figure out how numbers would work in your case.

The data for the airports, shopping centers etc. - you may just need to Google for it, or make estimates based on what you consider to be reasonable.

Some people also use POIs to create one-directional rush hour demand. For this you need work place POIs. Some are available on the workshop (e.g. Japan), others, you likely need to Google for places. Note the NIMBY residential data is really based on how buildings look like from the air, not actual population, so you can expect to have residential population in 100% industrial areas. So when using work POIs I just set up stop catchment area to minimal to get rid of the undesired population.

OK so; if it's a shopping centre; and I have a relevant demand curve, I'm sure I do I think I've downloaded all the ones on the workshop; and the internet says most recently that it gets 7 million people a year roughly; I'd need to divide that by 365 to get daily figures for the POI right? So use a relevant demand curve with a demand of roughly 19178 per day?

Yes for example you can set up 20,000 rounded. This is what you should expect to see if the center is open 24 hours. Since it's likely not (and not at 100% capacity), it will be something less than that. But in real life you would also expect some people will walk in (from home or from neighboring work places etc.), drive in, etc.

Note NIMBY demand is not two-directional. Generally you can often expect fewer people to go to the shopping center than originate from it. This is because as destination, it competes for people generated elsewhere with all other destinations. For this reasons sometimes you may want to set higher percentages on destination than on origin side. I have seen this dynamic with stadiums in particular but likely will be with shopping centers too.