I have a new laptop and an old one, it is possible to use the old laptop only as a second screen for the new one
preferably by cable because we don't have a good internet connection
Since a laptop or PC usually does not have a video input, this is not so easy.
However, you can retrofit a video input with a capture card, which then e.g. Connected via USB. You can also buy a monitor for the price of a reasonable capture card.
If both notebooks each have a free Ethernet connection (RJ45), then you could connect the two books directly using an Ethernet cable. https://wiki.ubuntuusers.de/PC-Direktverbindung_per_Netzwerk-Kabel/.
Then you would still need special streaming software, with the help of which you could then virtually expand your Windows desktop to the second book via streaming. https://www.borncity.com/blog/2016/03/29/spacedesk-zweiter-windows-bildschirm-per-netzwerk-teil-1/.
Direct video-out for the 2nd book is unfortunately not possible, as @ ChrisCat1 already explained to you.
It would be easier, however, to get a second monitor somewhere for small money, which you could then connect directly to your new notebook for two-screen operation.
