in attached sheet there is data in in merged and unmerged rows. How to join the texts in one cell from multiple cells, based on data at merged cells/ WITH THE HELP of formulae

Many Thanks @Excel Wizard
It works. It will be great if there is way to elaborate and understand this formula in step by step manner, with sheet
Thanks again

This is a 365 insider beta solution, so not generally available yet.

Code:

= LET(
filledDate, FillDownλ(date),
MAP(distinctDate#,
LAMBDA(distinct,
TEXTJOIN(",",,FILTER(text, filledDate=distinct))
) ) )
where FillDownλ is given by
= LAMBDA(x,SCAN("",x,LAMBDA(y,z,IF(ISNUMBER(z),z,y))))
and the distinct dates are
= FILTER(date,date)