I have one domain name but several webistes under it. Do I have to make a sitemap for every website that I have under my domain. For example, my main webiste structure is as follows

1) My main website is example.com and I have a sitemap.xml file in that directory
2) I have another site which is example.com/site2 Do I need another sitemap.xml in this directory as well. I have about 4 different sites like this scenario (site3, site4....).

The main sitemap.xml only has info about the example.com directory. My questions are:
1) Do I need a sitemap.xml file for every site.
2) Is there a way for me to just combine all the sites information into one sitemap.xml file and where should I upload it.
3) If I only need one sitemap for each site, do I upload all the sitemaps to the main directory with different names or do I upload them to each sites directory?

Thank you in advance.