The plugin QuickMapServices gives you access to many worldwide base layers, which can be easily added to your map from a menu.
Install from the Plugins menu > Manage and install plugins > search for quickmapservices > install plugin > activate by checking the box next the plugin name.
Access the plugin through the Web menu. By default, you have access to a few reliable base layers.
At your own risk, you can also access a wide variety of "contributed" base layers, of varying reliability and accuracy. Go to Settings > More Services > Get contributed pack. In my experience, the contributed layers are accurate, but they don't all work. Some are missing tiles, while others only work at certain zoom levels.
You can also manually add any layer for which you have a URL.
See the answer provided by Matthias Kuhn to this question (scroll down, it's near the bottom of the page) for help adding an XYZ tiles layer, and some URLs.
And see the answer to this question for help adding WMS layers.