Where does APKMirror.com get its apks? APKMirror.com hosts complete split apks, containing not just the base.apk but also many languages/multiple architecture support/vr apks etc. Take for example the contents of this chrome split apk:
https://www.apkmirror.com/apk/google-inc/chrome/chrome-88-0-4324-93-release/google-chrome-fast-secure-88-0-4324-93-5-android-apk-download/
.
├── autofill_assistant.apk
├── autofill_assistant.config.en.apk
├── autofill_assistant.config.ja.apk
├── base.apk
├── cablev2_authenticator.apk
├── chime.apk
├── chime.config.en.apk
├── config.ar.apk
├── config.de.apk
├── config.en.apk
├── config.es.apk
├── config.fr.apk
├── config.hi.apk
├── config.in.apk
├── config.it.apk
├── config.ja.apk
├── config.ms.apk
├── config.nl.apk
├── config.pt.apk
├── config.ru.apk
├── config.th.apk
├── config.zh.apk
├── dev_ui.apk
├── extra_icu.apk
├── image_editor.apk
├── image_editor.config.en.apk
├── stack_unwinder.apk
├── test_dummy.apk
├── vr.apk
├── vr.config.en.apk
└── vr.config.ru.apk

Where are they sourcing these from?
ApkMirror.com just accepts APK uploads from users. Anybody can extract APK files from their device and upload them to APKMirror.com. However it is not guaranteed that every upload is published. For ensuring security of uploading APK files each upload is automatically checked regarding it's cryptographic signature if it matches the signer of other versions of the same app.
The APKMirror community has set-up some rules about what APKs are getting provided for download and which not. For more details see their FAQ.
Therefore to answer your question the uploaded APK files can originate for any source. Some are extracted from devices (e.g. originally downloaded via Google Play Store), or the APK files are extracted from alternative app stores, web-sites.
In case of complete split APK bundles most likely they have been downloaded by PC based tools from Google Play Store (e.g. Raccoon4) and then uploaded to APKMirror.com. This can work because the Play Store protocol just provides links to all split files and the download client decides which parts to download.