This question follows from this one and one of its answers' recommendation to read Linuxtopia - Chapter 20. Subshells.

I'm a bit confused by this statement at the Linuxtopia site:

subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.
_{https://www.linuxtopia.org/online_books/advanced_bash_scripting_guide/subshells.html}

Does this mean that subshells, run from a script, are always run in parallel to the original script? Experimentally, this does not appear to be the case, but I'd be grateful for expert confirmation one way or the other.

#! /usr/bin/bash

# This script reliably prints:
# hello
# world
# 0
# ...implying that the the subshell is not run in parallel with this script.

(echo hello;
echo world)

echo $?

.

#! /usr/bin/bash

# This script prints:
# 0
# hello
# world
# ...implying that the the subshell is run in parallel with this script.

(echo hello;
echo world) &

echo $?

Is the use of & what the Linuxtopia site might have meant by "[letting] the script do parallel processing"?

This question follows from this one and one of its answers' recommendation to read Linuxtopia - Chapter 20. Subshells.

I'm a bit confused by this statement at the Linuxtopia site:

subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.
_{https://www.linuxtopia.org/online_books/advanced_bash_scripting_guide/subshells.html}

Does this mean that subshells, run from a script, are always run in parallel to the original script? Experimentally, this does not appear to be the case, but I'd be grateful for expert confirmation one way or the other.

#! /usr/bin/bash

# This script reliably prints:
# hello
# world
# 0
# ...implying that the the subshell is not run in parallel with this script.

(echo hello;
echo world)

echo $?

.

#! /usr/bin/bash

# This script prints:
# 0
# hello
# world
# ...implying that the the subshell is run in parallel with this script.

(echo hello;
echo world) &

echo $?

Is the use of & what the Linuxtopia site might have meant by "[letting] the script do parallel processing"?

Note: I'm familiar with the concept of suffixing commands with & in bash...it runs said command as a background process. So my question is more about whether command(s) executed in a subshell are run as background/parallel process by default, or if addition of the & here, as well, is what causes the background/parallel execution. The wording of the Linuxtopia article, to me, implied the former, which doesn't appear to match observation.