different_bash_commands

source is a bash shell built-in command that executes the content of the file passed as argument, in the current shell. It has a synonym in . (period).
Be careful! ./ and source are not quite the same.

./script runs the script as an executable file, launching a new shell to run it
source script reads and executes commands from filename in the current shell environment
Note: ./script is not . script, but . script == source script

What is sh
sh (or the Shell Command Language) is a programming language described by the POSIX standard. It has many implementations (ksh88, dash, ...). bash can also be considered an implementation of sh (see below).

Because sh is a specification, not an implementation, /bin/sh is a symlink (or a hard link) to an actual implementation on most POSIX systems.

What is bash
bash started as an sh-compatible implementation (although it predates the POSIX standard by a few years), but as time passed it has acquired many extensions. Many of these extensions may change the behavior of valid POSIX shell scripts, so by itself bash is not a valid POSIX shell. Rather, it is a dialect of the POSIX shell language.

bash supports a --posix switch, which makes it more POSIX-compliant. It also tries to mimic POSIX if invoked as sh.

sh = bash?
For a long time, /bin/sh used to point to /bin/bash on most GNU/Linux systems. As a result, it had almost become safe to ignore the difference between the two. But that started to change recently.

Some popular examples of systems where /bin/sh does not point to /bin/bash (and on some of which /bin/bash may not even exist) are:

Modern Debian and Ubuntu systems, which symlink sh to dash by default;
Busybox, which is usually run during the Linux system boot time as part of initramfs. It uses the ash shell implementation.
BSDs, and in general any non-Linux systems. OpenBSD uses pdksh, a descendant of the Korn shell. FreeBSD's sh is a descendant of the original UNIX Bourne shell. Solaris has its own sh which for a long time was not POSIX-compliant; a free implementation is available from the Heirloom project.

https://jvns.ca/blog/2017/03/26/bash-quirks/