heif-gdk-pixbuf and heif-thumbnailer are two different tools that can be used to work with HEIF images on Linux.
heif-gdk-pixbuf is a GDK-Pixbuf plugin that allows applications that use GDK-Pixbuf to display and manipulate HEIF images. GDK-Pixbuf is a library that is used by many applications to display images, so heif-gdk-pixbuf can be used to make a wide variety of applications able to display HEIF images.
Install heif-gdk-pixbuf:
sudo apt install heif-gdk-pixbuf