"followed by" definition

You have this mixed up. While it might seem logical say "the letter followed by C is B," in English usage, it's quite the opposite.

We would say:

"A is followed by B."

"B is followed by C."

Etc. Etc.

This comes down to some interesting ways in which our language (and our brains) interprets spacial/temporal order. If you are reading the ABCs from left to right, or singing them in order, A comes first and B comes next. To the listener or reader, it seems that B lags behind or FOLLOWS A. Ergo, in the passive voice, A is followed by B.

If you think spacially though it gets more complex. What is "front" and "back" in an ordered list of letters? We read left to right so you might say right=forward =front and left=backwards=back. From that, you might deduce B is in FRONT of A, A is BEHIND B, therefore A FOLLOWS B. That's not the way any English speakers I know think or speak, but it might be the source of your confusion.

The other part you are correct on. "Followed by" refers to the immediate successor in an ordered list. So while G comes after A just as much as B, you wouldn't say "G follows a" but rather something like "G comes later than A" or something like that.

Hope you've followed me!

There are two expressions which form a pair of opposites in this context. They are:

followed by

and

preceded by

The first means "what I am going to say next comes after what I have just said" and the second means "what I am going to say next comes before what I have just said"

For example "In the alphabet A is followed by B" and "In the alphabet B is preceded by A"

"Preceded by" is not used as often as "followed by"; not only because we tend not to talk about what comes before something as much as we talk about what comes after it but also because we tend to use the less formal "comes before" rather than "preceded by" when we do. However knowing that both expressions exist may help you to use "followed by" correctly.

Other tenses of the verbs "to follow" and "to precede" are also used giving expressions like:

"A precedes B", "C follows B" "A has B following it" and so on.

For items in a list "followed by" and "preceded by" normally refer to items next to each other so we would not say "C follows A in the alphabet" we would say "C comes after A in the alphabet" since B is between them.

This is different from the case where we are talking about people, for instance we might say "The suspect was followed by detectives for two days before he was caught red-handed". Obviously this does not mean that the detectives were immediately behind him all the time or he would have seen them and not committed the crime. However when we say "Obama was preceded by G W Bush and followed by Trump" the presidents have places in a list and this means that we can't say "G W Bush was followed by Trump" because Obama's presidency came between them. We would have to say "G W Bush came before Trump" but we could also say "Washington came before Trump" and be just as correct.