mjhjmtu

I am lost in the usage of pipeline

$ ls | echo
#does not return anything

alternatively, I tried

$ echo * | grep .md
2 Vocabulary.md a.out all_pdfs.md answer.sh area.sh ........while_count.sh while_menu.sh while_menu_2.sh

it return anything besides those not include 'md'

$ var=$(echo *) | grep .md $var
#return nothing

when I test

$ var1=$(echo *) | file $var1
2: empty
Vocabulary.md: c program text, UTF-8 Unicode text, with very long lines
a.out: Mach-O 64-bit executable x86_64
all_pdfs.md: empty
answer.sh: Bourne-Again shell script text executable, ASCII text
area.sh: Bourne-Again shell script text executable, ASCII text
badtest3.sh: Bourne-Again shell script text executable, ASCII text
bar.py: a /usr/local/bin/python3 script text executable, ASCII text
bash: directory
bashrc.md: ASCII text

and

$ var2=$(echo *) | ls -l $var2
ls: cannot access 'long': No such file or directory
ls: cannot access 'space': No such file or directory
ls: cannot access 'text.md': No such file or directory
-rw-r--r-- 1 gaowei staff 0 Apr 8 2018 2
-rw-r--r-- 1 gaowei staff 2036 Oct 19 10:04 Vocabulary.md
-rwxr-xr-x 1 gaowei staff 8464 Oct 29 15:05 a.out
-rw-r--r-- 1 gaowei staff 0 Oct 27 00:45 all_pdfs.md
-rw-r--r-- 1 gaowei staff 387 Apr 2 2018 answer.sh
-rw-r--r-- 1 gaowei staff 134 Apr 14 2018 area.sh
-rw-r--r-- 1 gaowei staff 467 Nov 2 16:46 badtest3.sh

How could I ensure in which situation pipeline will work?