awk 'FNR==NR{                                                                                     #### FNR==NR condition will be TRUE when first file named decimal.csv is being read.
A[++i]=$0;                                                                                        #### When above condition is TRUE then create an array named A with index of variable i whose index is increasing every occurance whenever above condition is TRUE, so means till first file is being read completly.
next                                                                                              #### skipping all other statements now by using next keyword.
}
{
for(j=1;j<=i;j++){                                                                                #### starting for loop now which will start only when second file will be read and wen first file will be completly read then we will get a final valur of variable i so running the loop from j=1 to till i's value.
split(A[j], array,",");                                                                           #### splitting the value of array A whose delimiter is comma(,). This array is having the values of decimal.csv files, which by splitting into array I wil use it later in code.
if(length($array[1])==array[2]){                                                                  #### Now I am comparing the value of field $array[1] which is field mentioned in decimal.csv and value is in file Input_file with length of field provided in decimal.csv file if they are equal then do following actions.
$array[1]=substr($array[1],1,array[3]) "." substr($array[1],array[3]+1);                          #### Here I am RE-FORMING the field let's say 2nd and 5th(as per your decimal.csv) file by using substr utility of awk. which works on substr(LINE,starting point, ending point) etc.
}
else{                                                                                             #### In case length of field is NOT equal to length provided in decimal.csv file then do following actions.
q="-" substr($array[1],1,array[2]);$array[1]=substr(q,1,array[3]+1) "." substr(q,array[3]+2);}};  #### creating a variable named q whose value I am constructing by field values by file Input_file, more important placing dot(.) here as per requirement in between field's values.
print                                                                                             #### finally printing the line, not here it will print the new formatted values of fields which we have done in above steps.
}
' decimal.csv  FS="|" OFS="|" Input_file                                                          #### mentioning first file named decimal.csv then Field separator as | and Output field separator as | for Input_file here.