pamac usescan access the AUR.
Like all AUR helpers it follows a PKGBUILD.
PKGBUILDS are build scripts.
Depending on the AUR packages PKGBUILD, if it depends on a deb, it may among other things, simply extract it.
pamac has no special deb powers. In fact it wouldnt know what to do with one on its own.
(makepkg on the other hand technically knows to treat debs as archives, but will only unpack the first layer … so many deb PKGBUILDS then need to decompress the data directory)
Bonus:
In the case of the google-chrome PKGBUILD, it uses bsdtar (on data like mentioned above);
https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=google-chrome