On command line bitcoin can be started in several ways
bitcoind starts a process with the current user. This way the home directory of this user will be searched for a config file and will be used as data directory as default.
service bitcoin start will use the startup script
/etc/systemd/system/bitcoind.service. This might contain a link to a configuration file. In my case this config file did not contain of a data directory which I found troubling. But the solution is that it contains a user. This user's home directory is then automatically referred to.
So, in my case I was using the terminal as user
pi and had to start the process as user
bitcoin instead. This can be done by
sudo runuser -l bitcoin -c 'bitcoind'