Extracting all the files into a directory is very easy:

using (FileStream fs = new FileStream("archive.zip", FileMode.Open))
using (ZipArchive archive = new ZipArchive(fs, ZipArchiveMode.Read))
{
    archive.ExtractToDirectory(AppDomain.CurrentDomain.BaseDirectory);
}

When the file already exists, a System.IO.IOException will be thrown.

Extracting specific files:

using (FileStream fs = new FileStream("archive.zip", FileMode.Open))
using (ZipArchive archive = new ZipArchive(fs, ZipArchiveMode.Read))
{
    // Get a root entry file
    archive.GetEntry("test.txt").ExtractToFile("test_extracted_getentries.txt", true);

    // Enter a path if you want to extract files from a subdirectory
    archive.GetEntry("sub/subtest.txt").ExtractToFile("test_sub.txt", true);

    // You can also use the Entries property to find files
    archive.Entries.FirstOrDefault(f => f.Name == "test.txt")?.ExtractToFile("test_extracted_linq.txt", true);

    // This will throw a System.ArgumentNullException because the file cannot be found
    archive.GetEntry("nonexistingfile.txt").ExtractToFile("fail.txt", true);
}

Any of these methods will produce the same result.