Recursive search for matches in two files

Question

On the second day, I'm trying to find a solution.
There is a CCC directory; we include subdirectories with numbers 1 2 3 ... 500, in each subdirectory there is a file named A1, containing one word (example) apple banana pear pineapple

$ CCC /
/1/A1.txt pear
/2/A1.txt apple
/3/A1.txt banana
/4/A1.txt pineapple

The flash BBB directory contains the ALLIN.txt file containing the following lines:

pear is a very sweet fruit
potatoes are very high in calories
pineapple number 1 for weight loss
raspberries tasty and healthy
everyone likes strawberries !!!

So my question is:
how to find and replace occurrences of the first word in a CCC file from a BBB file so that

CONCLUSION
$ CCC /
/1/A1.txt pear a very sweet fruit
/4/A1.txt Pineapple No. 1 lose weight

and files

/ 2
/ 3
delete

I had tried:

for i in `find ./ -depth -type d ! -wholename './'`
do 
join -j1 -o <${i}(sort A1.txt) <${i}(sort BBB.txt)
done

and

for i in `find ./ -depth -type d ! -wholename './'`
do
awk 'NR==FNR{a[$0];next} ($0 in a)' A1.txt BBB.txt    
perl -lane '$t+=1; $h{$F[0]}=$F[1] if $.==$t; print $F[0]," ",$h{$F[0]} if $t!=$.;$.=0 if eof' $(find -name '*A1.txt')  BBB.txt

I'm still learning Linux, so please, be kind to me.