When making a new character, you can enable Critical Role, Playtest, MtG, Eberron, and Rick and Morty content. What exactly do each of these add to character options and why are MtG and Eberron enabled by default?

Homebrew Content - when toggled on this allows you to use content you have created using the homebrew system. If you have a Hero or Master tier subscription you can add homebrew created and publically shared by others to your collection. Anything you have made or added to collection can be added/applied to your character if this is enabled.

Critical Role Content - enabled the Gunslinger martial archetype for the Fighter class, enables the the Bloodhunter class and enables content from Explorer's Guide to Wildemount. These are creations partly or wholly created by Matthew Mercer from the web show Critical Role where a bunch of nerdy-ass voice actors sit around and play D&D. D&D Beyond are sponsors of their second campaign.

Playtest Content - enables anything added to D&D Beyond as playtesting. This means some feats designed to compliment the Gunslinger, mentioned earlier, as well as select Unearthed Arcana content. Unearthed Arcana are playtest materials published by Wizards of the Coast and made available to public for free for them to test.

Magic: The Gathering - enables any content based off the card game Magic: The Gathering. This is a card game also published by Wizards of the Coast. So far only the book Guildmaster's Guide to Ravnica has been released.

Eberron Content - enables content from the books Wayfinders Guide to Eberron and Eberron: Rising of the Last War.

Rick and Morty - enables content from the Rick and Morty adventure.

"why are MtG and Eberron enabled by default?"

They are the only options that are only officially released sourcebooks - the others are either adventures (Rick and Morty), playtest or contain unofficial content (Gunslinger and Blood Hunter are not official content). Magic and Eberron are fully official content, but you have toggles because they're not set in Forgotten Realms like, almost everything else official is.

Homebrew toggle is the exception - it's on by default despite not being official, due to the popularity of using homebrew - if it was off by default people would complain at the inconvenience of having to keep switching it on.

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Hope this helps.

Thanks for the details! That helps a lot.

The thing that bothers me is that there isn't even anything to filter out from Rick and morty. That source has 1 magic item that is indistinguishable from anything in forgotten realms and 6 monsters that don't fit as well, but are monsters, so no one will notice.

Meanwhile, there is no toggle for AI content which includes 7 spells for multiple classes (most of which stand out as weird compared to other spells) and a CR0 non-legendary beast that is immune to damage (a druid can technically wild shape into this).

Quote from DxJxC >>

The thing that bothers me is that there isn't even anything to filter out from Rick and morty. That source has 1 magic item that is indistinguishable from anything in forgotten realms and 6 monsters that don't fit as well, but are monsters, so no one will notice.

Meanwhile, there is no toggle for AI content which includes 7 spells for multiple classes (most of which stand out as weird compared to other spells) and a CR0 non-legendary beast that is immune to damage (a druid can technically wild shape into this).

 
While I sorta agree, I do have to point out that the spells of AI may be weird, they are fun and useful. And as for the damage-immune beast, it is only immune in a single, specific encounter and outside of that it is considered a normal cat - easily killable. This is detailed in its description.

so what would I do for just like generic dnd characters , I'm gonna make something like a druid  build, even though I know I won't like ever use him, what would I do, as in what settings would I apply for that?

Generic? Probably none of them.

Quote from DataDwarf >>

My personal gripe (and this is probably small) is for subclasses like the Circle of Spores Druid that were originally published in a M:tG book and then republished in a core book, you have to have the toggle for M:tG enabled for it to show up.  There are probably spells, magic items, feats, etc that have this problem also, I just have not bothered to dig. I just make sure to always have the M:tG toggle enabled. Ideally, features that appeared in more than one source would be tagged as both sources and show up if either source was enabled.

 
I do agree it would be better for toggles to be clear on what you're actually toggling since it continues to change as new books are released.