Linux

Downloading all of wikipedia for one language is abiut 90GB. Inhave it on a spare drive in case of an outage. That way if I need to research something I can still do.

You can put together a media server and build a catalogue so you can watch movies and series offline. Maybe not a huge priority in that situation but definitely nice to have.

Jellyfin is a good option for streaming from a media server to other devices. The *arr suite is an option for building the catalogue.

Meshtastic

Navigation on Android: Osmand lets you download and cache OSM data so you can use it offline. Cache is unlimited if you download Osmand via F-Droid.

Spain? check guifi.net ;)

People had LAN Partys playing video games "offline" in the 90s... Setting up a network is easy, the difficulty comes from scaling up to many nodes, and spreading through the geography (e.g. if you were to use antennas for WLAN, they would need a mostly unobstructed vision) which in urban areas gets tricky.

But those "topology" issues can be flattened, e.g. you can always have a raspberry pi (or any device) acting as server in the corner of a neighborhood. A virtual bulletin board, emails, etc. all could be self-hosted locally there and then people could go grab a coffee and consume the local news just like in the middle ages, but with a screen, digital assets and some healthy amount of trolling :P

• Audiobookshelf: Audiobooks
• Navidrome: Music
• Jellyfin: Movies, videos, audio and books
• Radicale: calendar, contacts and tasks
• Nextcloud: all files and more
• HomeAssistant: for managing the solar panels, battery and other iot

This is going to be controversial but...

Linux is not really suited for the post-apocalitic no-internet world, the way the repositories are built and software is packed (almost nothing is static, a lot of dependencies on other packages everywhere) just makes it really impractical and hard to deal with those scenarios. Flatpak / containers and friends even make this situation worse because you can't easily mirror the repositories and there's no straightforward way of exporting a Flatpak as a solid file that can be shared around and installed everywhere - the current tool for that doesn't account architectures and dependencies very well.

Windows however is a much more solid and good option, yes, it's painful to hear this but in Windows you can get an exe from a friend in a flash drive and it runs as is. Same goes for installers, reinstalling the OS etc. There's only a couple of .net framework installers that will cover dependencies for 99.99% of stuff in a few MB. The same goes for macOS, however it depends on a lot of software signing nowadays and certificates that can expire and you then have a problem.

If you can get a few hundred watts of electrical power, StarLink is an option for broadband connectivity via satellite when all the local communications are down. Don't know why, but Star Link reminds me of Sky Net.

A piece of software always runs locally. It is in some cases those who needs to communicate with the server fail to deliver the usual function you expect when offline.

Please do not confuse one to another.

And perhaps you can start by complaining which services you are using heavily rely on the server side? General questions attract general answers and IMHO you are better off just search on the internet.