The one on the far left is a tomato. There are some other tomatoes as well. (If all your tomato seeds were the same variety, then their seedlings will look alike. But if you had different varieties of tomato, they might have very different-looking types of leaves. Most tomatoes have some version of "regular" leaves; some have "potato" leaves; others -- often dwarf varieties -- have "rugose" foliage; a very few have "angora" foliage.)

The tallest seedlings are tomatillos. You'll need at least two plants to have fruit.

I can't identify anything as definitely squash.

oh, there are about 8 or 10 tomatillo plants there. It's hard to believe that such a small seed is producing these sizable plants. I didn't realize I needed two together. Space them close together? Why is that?
What's the best way to separate this mess? It's a pleasant mess to have. Maybe, next batch, I'll be a little more careful where I put things.

the tomatoes are either roma or vine ripened.
The squash *might* be in this pic.

Yes, I see a squash just left of the middle. You can see one of its nearly-round seed leaves (aka dicotyledons).

Tomatillos need crosspollination to bear fruit. It doesn't have to be two plants of different varieties (as is necessary with many fruit trees), but simply two separate tomatillo plants. They don't have to be planted right next to each other: in fact, they don't both have to be in your yard, but the bees and other pollinating insects are more likely to get from one to the other if you have two plants in your yard: because who knows whether your neighbors will grow tomatillos this year or not?

As to how to separate them: first decide how many of each you have room for and want to plant. It looks like some are still sprouting -- and only one squash so far (?) -- so you might want to wait before proceeding. Then again, the longer you wait, the more entangled the roots will get, and the more seedlings may not survive being separated....

When the day comes for digging, take a good look at what you've got in the pot. Which are the strongest or largest seedlings? Which are growing off by themselves and will be easy to separate? Start with those. Maybe you'd like to mark them with a twist-tie looped around the lower stem.

Be prepared to sacrifice the ones growing too close, the smaller ones, the weaker ones, the extra ones.

Use whatever tools work -- perhaps a small trowel, maybe a plastic fork or spoon -- to dig up a seedling or group of seedlings.

If the clump of soil mix you dig up contains only one "keeper," then simply remove the others (you can cut them off at or below the soil line).

If the clump contains two "keepers" which aren't too close together, grasp each side of the soil mix with one hand and slowly pull them apart.

If it's a complicated clump with more than one "keeper," hold the clump of soil mix gently in one hand, grasp one of your target seedlings by its stem near the soil mix and gently wiggle it loose from the rest of the clump. If you're having trouble getting them apart, you may want to wash some of the soil mix away (though that can damage the roots).

Have some extras.

Plant the larger ones in their final location in the garden. Put the smaller ones (or the ones you're less sure of) in a small pot or plastic cup (with drain holes) for extra babying; transplant them to final locations when they're larger and stronger.

Particularly if the sunlight is strong, keep the ones in pots in shade until they perk up; it would also be a good idea to rig partial shade over those planted in the ground.

Good luck!

thank you for the suggestions.
I'm in San Diego, sun is always good here. Tmps here might hit the low 90s a few times out of the year.

Water the pot thoroughly before attempting to remove any plants. I would tip the whole pot onto its side and tip out the whole clump of soil onto a sheet of plastic or something. Easier imo to disentangle roots like that than by attempting to dig down.