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Some commands are built into Bash itself (builtins, like cd and echo); most are separate programs (like ls). They have different help systems.
This lab is the hands-on companion to the help lesson from Linux Journey (CC BY-SA 4.0).
help documents Bash builtins:
help echo
help cd
Most programs print a summary with --help. It's long, so page through it:
ls --help | less
type tells you whether a name is a builtin, an alias, or a program on disk:
type cd
type ls
type type
cd is a shell builtin
ls is aliased to `ls --color=auto'
help name documents Bash builtins.program --help prints a program's usage summary.type name shows what a name refers to.