How to make authentication for sFTP (ssh) user by key file?

Question

SSH

Hello!
Installed NGINX, folder with hosts /var/www/html/site.ru/.
Added to /etc/ssh/sshd_config at the end:

Match User ftp
ForceCommand internal-sftp
PasswordAuthentication yes // хочу сделать no
#AuthorizedKeysFile /home/danforth/.ssh/sftp_authorized_keys // путь к файлу с публичным ключем
ChrootDirectory /var/www/
PermitTunnel no
AllowAgentForwarding no
AllowTcpForwarding no
X11Forwarding no

The home directory for the ftp user is /var/www.
When I specify the path to the file, I get the following error in FileZilla:
Disconnected: No supported authentication methods available (server sent: publickey)
If I comment it back and enable password authentication, then everything is OK, it connects.
The file /home/danforth/.ssh/sftp_authorized_keysis ftp owner and group, and has read/write access only.
I also tried following this manual: https://wiki.archlinux.org/index.php/SFTP_chroot#F...
What could be the problem and how to specify the path to the public key file?
upd.1: Notice
the following in the logs:

Oct 16 12:52:17 quasar sshd[23847]: User ftp authorized keys /etc/ssh/authorized_keys/ftp is not a regular file

The current config is:

Match User ftp
ForceCommand internal-sftp
PasswordAuthentication no
AuthorizedKeysFile /etc/ssh/authorized_keys/%u .ssh/authorized_keys
ChrootDirectory /var/www/
PermitTunnel no
AllowAgentForwarding no
AllowTcpForwarding no

The path to the file is as follows:
/etc/ssh/authorized_keys/ftp/.ssh/authorized_keys
I tried to write the following in AuthorizedKeysFile:
/etc/ssh/authorized_keys/ftp/.ssh/authorized_keys
/etc/ssh/authorized_keys/ftp/
/etc/ssh/authorized_keys/%u/
It did not help.

Answer 1

In general, it worked with the following config:
namei -mo /etc/ssh/authorized_keys/ftp/.ssh/authorized_keys

drwxr-xr-x root root .
 drwxr-xr-x root root authorized_keys
 drwxr-xr-x ftp  ftp  ftp
 drwx------ ftp  ftp  .ssh
 -rw------- ftp  ftp  authorized_keys

In config:

Match User ftp
ForceCommand internal-sftp
PasswordAuthentication no
AuthorizedKeysFile /etc/ssh/authorized_keys/ftp/.ssh/authorized_keys
ChrootDirectory /var/www/
PermitTunnel no
AllowAgentForwarding no
AllowTcpForwarding no

In this version, it started up, apparently there was something with the rights.

Answer 2

1.The key file must be named authorized_keys.
I did not use the method described in the link.
2. access to the key should be only for the owner.