How can I synchronize the data on all devices without an internet connection?

Have a laptop with win7, a PC with win10, a smartphone with Android 9 with a ~ 400Gb memory card.

I would also have a second smartphone with Android 8, which would also be ideal if that were possible, but not all of the data because it has too little storage capacity.

I would like to save the data on the memory card instead of using the internal memory (of the smartphone, apart from that the internal memory is only 32Gb).

It doesn't work without the Internet. Synchronization depends on an internet connection. You would also have an online storage space, e.g. Need a cloud like Google Drive. And that costs money every month. With the amount of data at least 10 euro per month for 1TB e.g. At Google.

Of course it works without the internet, you just set up a Nextcloud server in the LAN.

Nextcloud server in the LAN. So you can sync back and forth.