Sub-string Extractor with Specific Keywords

JavaScript (ES6),  80  75 bytes

This contains some unprintable characters which are escaped below.

(s,a,b)=>s.replace(b||/$/,"").replace(a,"").match(/ *(.*?) *|$/)[1]||""

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Commented

(s, a, b) =>          // s = input string, a = start keyword, b = end keyword
  s.replace(          // replace in s:
    b || /$/,         //   look for the end keyword, or the regex /$/ if it's empty
    "\3"              //   and replace it with ETX (end of text)
  )                   //
  .replace(           // replace in the resulting string:
    a,                //   look for the start keyword
    "\2"              //   and replace it with STX (start of text)
  )                   //
  .match(             // attempt to match:
    /\2 *(.*?) *\3|$/ //   "\2"    STX
  )                   //   " *"    followed by optional whitespace
                      //   "(.*?)" followed by a non-greedy string (the payload)
                      //   " *"    followed by optional whitespace
                      //   "\3"    followed by ETX
                      //   "|$"    OR match an empty string to make sure that
                      //           match() doesn't return null
  [1] || ""           // return the payload string, or an empty string if undefined

Python 3, ^{86 77} 75 bytes

Saved 9 bytes thanks to movatica!!!
Saved 2 bytes thanks to ovs!!!

lambda s,a,b:s[s.find(a):(b in s)*s.find(b)if b else None][len(a):].strip()

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APL (Dyalog Extended), 24 bytes (SBCS)

Full program that prompts for array of [EndKeyword,StartKeyword,InputString]. Requires 0-based indexing.

⌂deb⊃(⌽⊢↓⍨1⍳⍨⊣,⍷)/⌽¨@0⊢⎕

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⎕ prompt for input

⊢ on that…

⌽¨@0 reverse all the elements that occur at offset 0

(…)/ reduce from the right using the following tacit function:

 ⍷ indicate with a Boolean list all the places where the left argument begins in the right argument

 ⊣, prepend the left argument to that

 1⍳⍨ find the offset of the first 1

 ⊢↓⍨ drop that many leading elements from the right argument

 ⌽ reverse (next time around, do this from the end, and after that, revert order)

⊃ disclose the enclosure caused by the reduction from a 1-dimensional array to a 0-dimensional array

⌂deb delete ending (leading and trailing) blanks