Search box

You can use the search box at the top of the Package Manager window to find any packages that match the text you enter. The Package Manager updates the package list as you enter text, displaying every package within the current scope that matches. For example, if you enter web in the search box, the list displays different results depending on whether you choose the Built-in packagesBuilt-in packages allow users to toggle Unity features on or off through the Package Manager. Enabling or disabling a package reduces the run-time build size. For example, most Projects don’t use the legacy Particle System. By removing the abstracted package of this feature, the related code and resources are not part of the final built product. Typically, these packages contain only the package manifest and are bundled with Unity (rather than available on the package registry).
See in Glossary scope or the All packages scope:

You can look for a package by its name or its version number. The Package Manager analyzes the text that you enter to detect which method you are using and updates the list of packages dynamically.

Note: You can search for either Unity packages or Asset packagesA collection of files and data from Unity Projects, or elements of Projects, which are compressed and stored in one file, similar to Zip files, with the .unitypackage extension. Asset packages are a handy way of sharing and re-using Unity Projects and collections of Assets. More info
See in Glossary. The filter you choose defines where the Package Manager searches:

Package name

You can search for either the display name as it appears in the Package Manager window or, for Unity packages, the registered package name that begins with com.unity.

Your search text could match the beginning, middle, or end of the package’s name.

Version number

Enter the version number in version order (MAJOR.MINOR.PATCH). For example, typing 1 returns all packages with versions beginning with 1 (for example, 1.0.0, 1.3.2, 1.10.1, and 10.0.0 all match but not 0.1.0).