steam is both a site and a program you install on your pc, it's an electronic store where you buy games and then download and play them, you buy games like with any e-store by using your credit/debit card, it has an option to remember the card number or not too so you can control when and how often he buys games if you want so he doesn't bankrupt you😅, avoid if possible the microsoft store cause i have heard many problems with it and their discounts aren't the best, but in general the most complete store with the most games is steam, then you have uplay, gog, origin and epic, my advice is to install them all as they all give freebies from time to time, yes not all games are great but it doesn't hurt to try them, half the games i have were given for free
another thing, you should prefer digital for games because most pcs don't have cd/dvd player anymore and you would have to buy it extra, it's not expensive but it's unnessecary hassle and you have the boxes/discs taking up space