Interjections have no grammatical association with a sentence in which they are used. This doesn’t disqualify them from being used frequently though. They are found more often in speech than in writing. However, fictional writing may use a lot of them when directly quoting the characters.
And so on. As you can see, the words are short exclamations which we use in expressing greetings, surprise, pain, joy, etc. They may be followed with a comma or an exclamation mark (!) to indicate a stronger emotion.
Ah: Used to express pleasure, surprise, realization, etc. “Ah! I finish first!” “Ah, now I get what you’re saying.” “Ah, that feels relaxing.”
Alas: Used to express grief. “Alas, we lost the finals.”
Well: Used to express surprise, bring forth a remark. “Well, what happened next?”
Er: Used to express hesitation. “Er…I am not sure, is it correct?”
These are all emotional expressions. Formal academic writing rarely uses one except for quoting someone in direct speech.