It is sad the revelation of his involvement came so close to the end of his life. He certainly deserved much more recognition.
One-time pad is the simplest, yet secure symmetric encryption algorithm, i.e, you have a 128-byte plaintext, you can take a 128-byte random string from your one-time pad, XOR it with your plaintext to encrypt it. As long as the random string is not shorter than the plaintext and not reused, the encryption is information-theoretic secure.
However, an one-time pad is often too problematic to be practical. First, the one-time pad has a large size (it needs to be larger the entire plaintext you want to encrypt), which is difficult to be stored and shared securely, also, the one-time pad needs to be created from a physical process, which takes time.
But if you have an algorithm to generate random number, you can just initialize it with a random seed, then the it starts to produce random strings of indefinite length, which can be used to create an one-time pad. Since the RNG algorithm is deterministic, only the short seed is needed to be shared between Alice and Bob. Subsequently, they can create their own copies of the identical one-time pad by running the algorithm with the same random seed.
Now, the one-time pad encryption algorithm is still the same one, the only difference is the pad is generated from a software random number generator, instead of a true random process. Therefore, the security of the encryption is reduced to the security properties of the random number generator. If the RNG is completely unpredictable and cryptographically secure (aka CSPRNG), it would be as good as true random strings.
You can take the step further, wrap the CSPRNG and the XOR function in a blackbox, call the blackbox a "cipher", and call the random seed of this CSPRNG as the "key", you've just created a "stream cipher".
In other words, the core of a stream cipher is a CSPRNG, which is used as an approximation of one-time pad. Historically, RC4, and now ChaCha, is widely used for both purposes.
When the CSPRNG does not produce uniformly-distributed random strings, bad things happen. RC4 is weak because of the existence of multiple types of biases in its random stream, same bytes appear more often that others.
If you traveled 300 years back in time, the first thing you would need to work on is a CSPRNG. When you have one, a strong stream cipher naturally appears. Luckily, you can make a random number generator from various forms of common permutations, like a deck of cards. https://en.wikipedia.org/wiki/Solitaire_(cipher) If you use this algorithm, your communication would be safe for 100 years, as the cryptanalysis tools required to exploit the bias didn't exist from that time. Or even better, use the LC4 cipher, https://news.ycombinator.com/item?id=16586257, which the random number generator can be obtained by making a simple toy.
Tunny (the German thought of this as "Lorenz" because that was the manufacturer name) is an actual stream cipher, more or less. It's making pseudo-random bits and XORing them with plaintext to produce a ciperhtext. The plaintext is Baudot code rather than, say, ASCII, but it's definitely bits. Lorenz is doing this in (by modern standards) a ludicrously mechanical steampunk-looking way, but that's what it's doing. Even though to our modern eyes it looks not so different from Enigma in terms of the principle of operation it's entirely different and radically more like how things would be done decades later.
Salsa20 and ChaCha are modern stream ciphers, they're designed for modern binary computers rather than a mechanical contraption, but the core underlying principles are the same as Lorenz. If you can make a lot of "random-looking" bits from a small key you can XOR them with your plain input to produce a ciphertext, nobody can decipher it without knowing the key in order to generate the same "random" bits.
Most cryptosystems you use today have block ciphers inside them, which work quite differently, but some will have a stream cipher, a descendent of Lorenz inside them - spiting out "random-looking" bits.