hi,
there is no one who got expertise in the earlier stage. we all here to learn.
so let's start
for the top menu:
refer:
http://prntscr.com/r3ze1n as you can see that you can set any page/post/custom link in your menu.
the top menu will reflect here:
http://prntscr.com/r3zfz3
you can also provide a custom link in the menu:
http://prntscr.com/r3zm9h
for the main menu:
refer:
http://prntscr.com/r3zn6t selected as the main menu below. you have already done the same. which will appear here:
http://prntscr.com/r3zo88
for the footer menu:
refer:
http://prntscr.com/r3zp2m which will appear here:
http://prntscr.com/r3zpi4, as you can see here there, is another option in the menu to set as the mobile menu so you can select/create a menu and set as mobile menu then it will appear in the mobile. it's just as simple.
you can also create a mega menu for the mega menu you can make use of this tip:
https://wplms.io/support/knowledge-base/setting-up-mega-menu-in-wplms/
and for another one query: as you have mentioned sub-domain in your query then you can make use of wplms multisite.
please refer to this tip:
https://www.youtube.com/watch?v=i80MKz2YcfY