Using several variables in file, based on lines in other files

Question

First post ever. I'll do my best :). I have no coding skills whatsoever, but am trying to learn, and have dabbled a bit in Linux.

So, I have a master document (master.txt). In this document, I would like to have a text including several variables, which are linked to other files (a.txt, b.txt, etc.), which can be changed manually. All this should produce a whole bunch of files.

So, lets say a.txt contains the following lines of text:

Mary

John

b.txt contains the following lines of text:

England

The US

Australia

master.txt contains this text:

"Hi there a, do you live in England?"

Now I would like to export several files at once, resulting in:

master_a1.txt: "Hi there Mary, do you live in England?"

master_a2.txt: "Hi there John, do you live in England?"

Then I'd slightly edit master.txt to:

"Hi there Mary, do you live in b?"

To produce:

master_b1.txt: "Hi there Mary, do you live in England?"

master_b2.txt: "Hi there Mary, do you live in The US?"

master_b3.txt: "Hi there Mary, do you live in Australia?"

I've been looking, searching and trying but cannot figure it out. The closest I came was based on something here on stackoverflow:

count=0
while read -r number; do
    count=$(( count + 1 ))
    printf 'Hi there %s, do you live in England?' "$number" >"/home/x/export/master_a"
done < ~/x/a.txt

Any help would be much appreciated! :D