Confusing with the area of DID with multiple time periods

[Kunnummal Abdurahman Rahees]

Hello all,
I am doing research based on firm-level unbalanced panel data. Here, I am working on the de-reservation policy impact on the performance of the small-scale manufacturing sector.
The reservation policy is the policy of the Indian government that certain products were exclusively manufactured by small-scale industries from 1967. It was highly criticized by the researchers that it decelerates the overall manufacturing sector growth in India. Finally, the de-reservation of these products started gradually in 1997 and ended in 2015.
Here I plan to analyze incumbent small firms' performance using unbalanced panel data from 2001 to 2020. Here I plan to use the CSDID command in Stata that determines difference-in difference with multiple time periods.
My data looks likes this:

* Example generated by -dataex-. For    more info, type    help    dataex
clear
input str8 Factory_id int Year float    yearderesfirst
"0000003F" 2002    0
"0000006F" 2010    0
"0000006F" 2017    0
"0000008B" 2019    0
"0000008B" 2020    0
"0000022F" 2017 2007
"0000023F" 2017    0
"0000033F" 2005    0
"0000033F" 2010    0
"0000033F" 2018    0
"0000103F" 2003 2005
"0000103F" 2004 2005
"0000103F" 2011 2005
"0000103F" 2018 2005
"0000106F" 2007 2006
"0000106F" 2014 2006
"0000106F" 2015 2006
"0000108B" 2006    0
"0000108B" 2013    0
"0000108B" 2015    0
"0000108B" 2019    0
"0000110F" 2013 1997
"0000129F" 2005 2008
"0000132F" 2018    0
"0000132F" 2019    0
"0000133F" 2003 2007
"0000133F" 2009 2007
"0000133F" 2010 2007
"0000133F" 2015 2007
"0000206F" 2007    0
"0000206F" 2008    0
"0000206F" 2009    0
"0000206F" 2010    0
"0000206F" 2011    0
"0000206F" 2013    0
"0000206F" 2014    0
"0000206F" 2015    0
"0000206F" 2016    0
"0000206F" 2017    0
"0000206F" 2018    0
"0000206F" 2019    0
"0000206F" 2020    0
"0000208B" 2006    0
"0000208B" 2007    0
"0000208B" 2008    0
"0000208B" 2009    0
"0000208B" 2010    0
"0000208B" 2011    0
"0000208B" 2012    0
"0000208B" 2013    0
"0000209F" 2003 2015
"0000209F" 2005 2015
"0000209F" 2013 2015
"0000209F" 2019 2015
"0000209F" 2020 2015
"0000219F" 2004    0
"0000219F" 2020    0
"0000220F" 2014    0
"0000220F" 2015    0
"0000222F" 2018    0
"0000233F" 2005    0
"0000233F" 2009    0
"0000233F" 2011    0
"0000306F" 2007    0
"0000306F" 2008    0
"0000306F" 2014    0
"0000306F" 2017    0
"0000308B" 2007    0
"0000308B" 2011    0
"0000308B" 2012    0
"0000319F" 2004    0
"0000319F" 2005    0
"0000322F" 2019    0
"0000323F" 2020 2008
"0000333F" 2006 2007
"0000333F" 2011 2007
"0000333F" 2013 2007
"0000333F" 2017 2007
"0000406F" 2007 2008
"0000406F" 2008 2008
"0000406F" 2017 2008
"0000407F" 2019    0
"0000407F" 2020    0
"0000408B" 2006    0
"0000408B" 2010    0
"0000408B" 2016    0
"0000408B" 2017    0
"0000408B" 2018    0
"0000408B" 2019    0
"0000408B" 2020    0
"0000419F" 2004    0
"0000419F" 2005    0
"0000420F" 2011 2007
"0000420F" 2019 2007
"0000420F" 2020 2007
"0000424F" 2001    0
"0000424F" 2003    0
"0000424F" 2007    0
"0000429F" 2001    0
"0000429F" 2011    0
end

Note that here the variable "yearderesfirst" first is not zero for all the years. It varied between 1997 to 2015. I don't have any issue with the CSDID stata commands.

My doubts are:

1. Is it possible to include the data after 2015? because the de-reservation ended in 2015.
2. How can we interpret CSDID results? Are there any reference materials available?
3. Do we need to use covariates for the CSDID regression? Because I am getting the results without covariates.

Your valuable help makes lots of difference in my research.
Thank you