For app developers, there's a lot of benefit in having their software start up with macOS: It means the program is available at a moment's notice, can carry out background tasks (such as updates) while you're busy on something else), and is able to ping you with a notification or two whenever it likes.
This is fine, up to a point—for the apps you genuinely need and use a lot of the time. But when too many of these apps are automatically starting up with macOS, you'll start to notice the impact on performance. You don't necessarily want a bunch of programs running all the time and sitting in memory.
As on Windows, you've got full control over which apps launch with macOS: You can add your favorites, and remove the programs you use less often (you can still launch them in the normal way, as and when needed, of course). It's worth checking this list regularly too, to check for software that's sneaked in without your knowledge.
Before you get started, you might want to see what's running in memory right now on your Mac, and what's using up your computer's precious system resources. Launch Activity Monitor from Spotlight or the Utilities pane in Finder to get this information—you can close down any process by selecting it, then clicking on the X in the toolbar.
It's a good idea to check the settings inside individual apps first: This can take longer than loading up macOS System Settings, but it avoids any potential conflict between programs that think they're allowed to boot up with macOS, and an operating system that doesn't want them to.
The setting will be in different places in different programs, but you should be able to find it without too much trouble—if it's there. In the Spotify desktop app for macOS, for example, click on your profile picture (top right) and Settings, and you'll see a drop-down menu labeled Open Spotify automatically after you log in to the computer.
A lot of apps that start with macOS will take up a perch in the menu bar, which can be a good place to find the settings to turn them off. Take Dropbox as one example: Right-click on its menu bar icon, choose Preferences, and on the General tab there's a toggle switch marked Open Dropbox upon startup (you'll see every app calls it something different).
When you've been through all the individual software settings you can find, open up the Apple menu, then System Settings. From the General tab, choose Login Items & Extensions: The next screen shows everything that's launching with macOS, and all the processes that have permission to run in the background (even when the apps they're associated with aren't running).
Removing an item from the login list is as easy as selecting it, then clicking the - (minus) symbol—it'll disappear without any additional fuss. To add something to this list, so the program is always easily available for you in macOS, click the + (plus) button, then find and select the app on your system.
Further down the same dialog, you can opt to disable some of the background processes starting up with macOS as well, if needed: These are usually tools for checking for updates for their relevant apps. Ideally, though, you want to find the settings for these apps in the apps themselves, as playing around with the toggle switches here can sometimes cause software to misbehave.
