I can only try-
The advantage to the method you mentioned in your opening post is that it can be decoded by hand.
I read a story in an old book about the FBI from the mid 1950s (sorry if that makes you old) about a paper boy who got paid a Nickel that felt different. It turns out that it was hollow with the front and back being from two different years. Anyway, inside the nickel there was an encrypted message that the FBI couldn’t figure out. (They knew it was from the Soviets from the precision that the two halves fit together.)
When they finally got to the bottom of the case it turns out that it was encoded with a similar technique that you mentioned above.
Modern encryption nowadays uses mathematical functions that have no inverse function. Public Key/ Private Key methods uses prime numbers. While there are various methods to test the primality of a number, one will only know if it is prime or not, not what the factors are. One can easily take two long primes and multiply them together while it could take a hacker months or years to factor the product.
(The details on how this works is beyond the scope of this thread. If you are interested in a real life working example please let me know.)
Another method I used to use when I was younger is Matrix Encryption, and this can be done by hand. The way it works is an invertible 3 X 3 matrix is used to encode a message and the message is decoded by finding the inverse of the matrix and multiplying it by encoded message.
When my younger brother was in camp he mentioned that he thought it was nosy that the office staff would read the faxes that came in so he asked me to encode them. At the beginning of each fax I would tell him which matrix to use. He loved all the attention he was getting as his bunk would watch him meticulously decode the message claiming it was extremely sensitive information.
He had to leave camp a few days early that year so on his last day I send a fax that said “Urgent, Please deliver to Moshe Pepper Immediately.
Our Cipher has been compromised, abort mission immediately and leave camp at once… He got a kick out of it.