Open-source alternatives to Buffer

Postiz and Mixpost are open source: free to download and run on your own server. Buffer costs from $5 a month, with a free plan.

Postiz

Open source (AGPL-3.0) · Open-source social media scheduling and analytics

Postiz schedules, automates and analyses social media posts. Use Postiz Cloud or deploy the open-source edition yourself.

Free to download and run on your own server under the AGPL-3.0 licence; you pay only for your hosting.

Open source (MIT) · Self-hosted social media management

Mixpost plans and schedules social media posts and supports team collaboration. You install it on your own server.

Free to download and run on your own server under the MIT licence; you pay only for your hosting.

Free to use, not free to run. The licence costs nothing, but self-hosting needs a server (often $5 to $40 a month) and someone to install updates and keep backups.
Open core means the main product is open source but some features, usually for larger teams, need a paid licence.
Hosted versions exist for many of these, run by the makers. They cost money but save the upkeep, and you can still move to your own server later.
Licences differ. MIT and Apache allow almost anything. GPL and AGPL require you to share changes you make to the code; for normal business use, that rarely matters.