https://airesistlist.org/ (currently down: see the Internet Archive) has links to several concrete projects, including https://di.day/en (see also, https://european-alternatives.eu/, https://switching.software/). The IndieWeb (https://indieweb.org/) and the Fediverse[1] are both movements to take back parts of the internet. We can all make small changes in our lives: what would you do, if you refused to acknowledge the existence of Big Tech, and needed to choose another approach? And can you do this, in practice?

Low-tech Magazine (https://solar.lowtechmagazine.com/) has enough articles to publish a thematic book, “How To Build a Low-tech Internet?”. There are other books with concrete proposals: pretty sure Cory Doctorow's published a few (Chokepoint Capitalism, also by Rebecca Giblin; Enshittification; possibly others). You can read these if you would like. But the important part is using and maintaining credible alternatives, reducing both our dependence on and support of these companies.

[1]: although more work is needed to reduce the addictiveness / increase the user empowerment of Fediverse UIs (which are largely modelled on the corresponding Big Tech social media systems): websites are still the way to go, if you can.