What if AES-128 key / text length is more or less than 16 characters In this example ( https://kavaliro.com/wp-content/uploads/2014/03/AES.pdf ), the AES-128 key and text are 16 characters each, but how would this example be explained if the key or text was more/less than 16 characters? How would this change the procedure that creates the encrypted text?
AES is not a method of encrypting messages with passwords.  It is a pseudorandom permutation family, which is not designed to be used directly by applications.  Rather, a PRP is a very specific technical concept that cryptographers use to design methods of encrypting messages, called authenticated ciphers, like AES-GCM—and that cryptographers could use to derive keys from passwords, though usually we use other password-based key derivation functions like PBKDF2-SHA256 or scrypt or argon2id that don't involve AES.

For example, you could use scrypt to derive a 256-bit key $k$ from your password, and then encrypt your message $m$ using the authenticated cipher AES-GCM under the key $k$—or the even better authenticated cipher crypto_secretbox_xsalsa20poly1305 from NaCl which doesn't involve AES at all.

Don't use the letters A-E-S directly in your application; use an authenticated cipher.  The keywords above should lead you to plenty of followup reading material.