Below query is used to get Apex Classes names in Salefsforce.

SOQL:

SELECT Name, Body FROM ApexClass

One more approach is use Ant tool to fetch all Apex Classes with the below Package.xml.

  <types>
        <members>*</members>
<name>ApexClass</name>
    </types>

To get the names of the retrieved Apex Classes, use the below link

https://www.infallibletechie.com/2014/10/how-to-copy-all-files-names-from-folder.html

Cheers!!!