how to get column names dynamically when expanding tables?

While it is technically possible, it is quite a resource expensive approach to read all the headers from all the tables to then create a custom list of headers that you might want to expand.

You can try and leverage an approach that uses the following functions:

• Table.Column
  • You can use this function to extract the column where you have the table values from your table and then have a list of tables that you can use with List functions
• List.Transform
  • This is a common List function to help you transform the values inside of a list based on your specific logic
• Table.ColumnNames
  • This is a function that effectively extracts the column names from your table and the output is a list of strings (the strings being the column names)
• List.Combine
  • Similarly to how the "Append" operation works to combine multiple tables (it uses Table.Combine behind the scenes), List.Combine "Appends" lists into a single list
• List.Distinct
  • Once you have the values of all list of columns into a single list, you will end up with a lot of duplicate names for the column names. This function will help you get rid of the duplicates

There are other approaches, but they depend on what your requirements. I'm sharing the links below as other approaches that I've used throughout the years:

https://www.youtube.com/watch?v=XPWnPAhQEiQ

https://www.youtube.com/watch?v=omu-sjzNO44

Hope this puts you in the right path!