I have 16GB of RAM with Debian 9 in my notebook but almost all of it is unused by the kernel. It uses only 1GB for buffer/cache.
Where can I tell the kernel to use more of the free RAM for caching? My FS is Ext4.
The kernel only caches data that has been accessed. If you have not read more than 1GB of data from the disk since the last boot, then it will not have more than 1GB cached.