"apt-mark showmanual" will show you all packages that you have manually installed
"apt-mark showauto" will show you all packages that are automatically installed
"apt-mark showinstall" will show you all the packages that are installed
How is this useful? It tells you what packages are installed in your system.
So you can go "apt-mark showinstall | grep somelibrary" to see if you have a particular library installed that's a dependency.
WD HDD Ultrastar DC HC520 HUH721212AL4205 12TB 3.5" SAS Server Hard Disk Drive
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Seagate Exos 7E10 ST2000NM000B 2TB 7200RPM SATA 6.0Gb/s 3.5" Internal Hard Drive
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HDD 3.5" SATA Hard Drive with Windows 7/Win 10 Installed Legacy
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Seagate Exos X14 12TB SATA6Gb/s 7200RPM 3.5" Enterprise Hard Drive ST12000NM0558
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