Some Fediverse alternatives to "big tech" social media:
Twitter ➡️ @Mastodon
Instagram ➡️ @pixelfed
YouTube ➡️ @peertube
Twitch ➡️ @owncast
Goodreads ➡️ @bookwyrm
Facebook Groups ➡️ @mobilizon
Medium ➡️ @writefreely
You don't need an account on all of these in order to interact with them.
Because they all use the ActivityPub open protocol, you can (for example) use a Mastodon account to follow someone on PixelFed or vice versa. That's why they are collectively known as the Fediverse, because they federate together at a technical level.
Also, this is an incomplete list, there are many other Fedi projects out there such as @funkwhale or @inventaire that are not direct alternatives but are their own thing.