How does salt get used in openssl passwd hash (md5)?

Question

I was trying to manually generate the password hash stored in /etc/shadow (just for curiosity).

For example using salt a and password a with md5 hash algorithm, with the command

openssl passwd -1 -salt "a" a

i get

$1$a$44cUw6Nm5bX0muHWNIwub0

Where 1 correspond to md5 hashing algorithm, a is the salt and 44cUw6Nm5bX0muHWNIwub0 should be the actual hash.

I was expecting to get the same result calculating the md5 hash of aa (password and hash concatenated), and converting it in B64 (as explained on wikipedia) but I get a different output.

What is the right way to hash and encode the password and the salt?