In HTTP specification, what is the string that separates cookies?

The answer is a comma , sign.

In section 4.2.2 of RFC 2109 there's this specification of Set-Cookie header

   set-cookie      =       "Set-Cookie:" cookies
   cookies         =       1#cookie

with the following statement Informally, the Set-Cookie response header comprises the token Set-Cookie:, followed by a comma separated list of one or more cookies. (Formally meaning of # in the above notation is defined in RFC 733 in section A. NOTATIONAL CONVENTIONS, point 5

A construct "#" is defined, similar to "*", as follows:

              <l>#<m>element

indicating at least <l> and at most <m> elements, each separated by one or more commas (",").

Yes, RFC 2109 was obsoleted by RFC 2965, which in turn was obsoleted by RFC 6265.
No, it doesn't change anything in this context as

• most existing HTTP servers and clients support RFC 2109
• RFC 6265 does not forbid Set-Cookie folding