use id to find out about user and group info. cat /etc/passwd to find out about other users.
hostname to find out pc name. cat /etc/issue and uname -a will both display the OS info.
ps axu to get info on running processes. ifconfig a to show IP config info. route/routel to show routing table. netstat -anp to show active connections.
to see firewall usually need to have root (iptables). However can check cat /etc/iptables also to see if got any info there.
use ls -lah /etc/cron* to find out interval scheduled jobs (hourly, weekly etc). cat /etc/crontab to find out custom tasks.
use dpkg -l, apt list --installed or rpm -qa to list installed packages.
find / -writable -type d 2>/dev/null to find out folder with write permission (use f for file) by current user.
use both mount and cat /etc/fstab to find out info on mounted drive. /bin/lsblk to see all available drives.
use lsmod to see list of modules, then /sbin/modinfo modname to get specific info.
use find / -perm -u=s -type f 2>/dev/null to find file with SUID bit.
Check for common backup file location, such as: ls -la / ls -la /tmp ls -la /var/backups/ ls -la /home/user/