Requesting Some Help using Multisite for the First Time

I currently have 14 sites that I would like to combine using MS. So on a sub test folder I installed WP and then edited the config and ataccess files as instructed. Under this new folder I have added sites as instructed.

Questions:

1) The sites do not have WP on them, shouldn’t they be using the main WP install.

2) Consider I am combining existing domains under one multisite – can I keep the old URL? is there a tutorial on this somewhere.

3) Also is there an easy way of moving old data from the current url into the new site in the new MS. or is exporting/importing the only option.

4) Can I use the old site URL name in the MS (for SEO purposes) or do I have to change that too. so I guess it is subdomains vs urls?

My Main Site is:
https://www.Brasil-USA.com

I would like my subdomains to ideally be:
https://www.brasil-miami.com vs. https://miami.brasil-usa.com

Not sure it this is possible.

Lastly where is the option set if I choose subdomains or suburls in the MS setup.

Thanks to anyone that can help answer my questions!!