Which command show memory utilization?

Which command show memory utilization?
free
Which command show list of block devices?
lsblk
How to check folder size in Linux?
du
How to print routing table?
route -n or ip -r
Which command show information about network card driver and firmware?
ethtool
How to check number of CPU available for operation system?
nproc
How to check load average?
any from that list: w
, who
, top
, htop
What does file permission 0600 means?
Read and Write operation allowed only to file owner
How to find list of open files in system?
lsof
What does HTTP response code 200 means?
OK, success