Is it possible to manually execute the command /usr/libexec/openssh/sftp-server and try to write some command or text that allows me to know if it works? for example, list directories.
Where can I find documentation on the openssh/sftp-server protocol after ssh login?
SFTP is a binary protocol, defined by IETF standard draft. Latest version can be found on IETF website: https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-13, but make sure it's what your server supports.
As it's a binary protocol, you would need a specialized client program to use it.