A one time pad is a specific case of substitution cipher (it’s a generalization of vignere) where the key is the length of the document. It is probably secure - as in it is actually impossible to break.

The reason one time pads are not used in general is that you need a “perfect” rng, and you have to be able to get the random values to the recipient. Those old “person traveling with brief case of secrets” trope was a real thing. Key distribution is the problem solved by public key cryptography. But you can’t use one time pads with public key crypto, because the weakness is then breaking public keys (which is probably possible).

Stream ciphers loosely acted like a one time pad in that you generate a “random” stream and xor with the message. But it doesn’t reach the actual requirement of security for a one time pad because the key is the RNG seed, which means you can brute force the seed key space and only the correct key will produce a completely sensible decrypted output.

A true one time pad means that a brute force search of the key space for a message of length N will find every valid message of length N.

Eg an 11 letter message would produce (among others) “hello world” and “hello earth” as well as “die planet!”.

It is true for any cipher, but remember that you are not allowed to reuse the key. If you are just scrambling the alphabet, you can never encrypt more than a single character without key reuse.

Two different techniques here. One-time pad is a strong random cipher, versus a typical "lookup" substitution cipher which is garbage.