about the free domain

I already have a domain name with a registrar which is paid for the next five years. Firstly, do I have to do much to transfer that to point from my current host to you?

secondly, would they be able to still take advantage of the free domain? In fact, I probably won't have any particular domain I want to set up with the free domain at the moment, but maybe once I get my old domain settle down a may think of something I want for the new one

If you display at the order that you want a FREE domain name you will get it, and also you will be able to point your existing domains to your accounts.
But if you have mentioned at your order your existing domain name you will not get FREE domain - we will only transfer your domain name from your current registrar to our if you'll wish.

we must choose free domain when we join then?

Yes, you are right

what about sub domains?

Do sub domains only exist for the free domain were we join, or do they also work for extra domains that we park.

what if we want to make for example

bob.freedomain.com
and
jane.pre-existing.com

you can add unlimited subdomains for any domain set up in your control panel

if we make

bob.freedomain.com and jane.pre-existing.com

then will
jane.freedomain.com and bob.pre-existing.com also automatically exist? or not.

or does it always redirect to freedomain.com

Yes you'll be able to create unlimited subdomains like one.freedomain.com two.freedomain.com. etc *.freedomain.com

Can I have bob.freedomain.com but NOT bob.pre-existing.com?

No. There will be only bob.freedomain.com and jane.pre-existing.com but not
bob.pre-existing.com or jane.freedomain.com.