Self-hosted instance of FreshRSS [0], Readrops [1] as the client on Android, html2rss [2] for websites that don't have RSS feeds and AtoMail [3] to get newsletters as RSS feeds instead.

Edit: forgot to mention RssBridge [4] to get tweets that match certain criteria as an RSS feed. RssBridge also provides access to many other platforms.

[0]: https://github.com/FreshRSS/FreshRSS

[1]: https://github.com/readrops/Readrops

[2]: https://github.com/html2rss/html2rss-web

[3]: https://github.com/remko/atomail

[4]: https://github.com/RSS-Bridge/rss-bridge