There’ve been apps to use them as a second monitor to a Mac for some time, but as of a few years back, it’s a built-in feature.
Last I checked it does require the iPad to be signed in to the same iCloud account as the user on the Mac, which may present a problem if, say, you want to use a personal iPad this way with a work MacBook, or if you use separate accounts for different clients on your MacBook and don’t have them all signed in to a single iCloud account. But, if that’s not an issue, should be fine. Some of those other apps may still exist, too, and I don’t think they have that restriction (I think Apple’s is that way because it’s more tightly integrated with hand-off features and such)
Last I checked it does require the iPad to be signed in to the same iCloud account as the user on the Mac, which may present a problem if, say, you want to use a personal iPad this way with a work MacBook, or if you use separate accounts for different clients on your MacBook and don’t have them all signed in to a single iCloud account. But, if that’s not an issue, should be fine. Some of those other apps may still exist, too, and I don’t think they have that restriction (I think Apple’s is that way because it’s more tightly integrated with hand-off features and such)