I learned how to unstack items, But how do you restack them instead of separating them?

When the items have previously been unstacked, to restack them, you'd click one item, then another of the same kind, and it'll once more stack them together.
There's no button to restack a group like there is to separate a stack.

The only exception to this is if you have multiple of the same item, but some are tradeable and others are non-tradeable. Those will each have their own stack of tradeable and non-tradeable, but despite being the same item, their ability to be traded will prevent them from stacking together.
Their stacks can still be separated though.

How do I split stacks? Hope you don't mine me hijacking your thread haha, but can't figure it out.

Hi! If you're on mobile or desktop, you can split stacks by following these directions:
- Click the stack of the item you want to split.
- Click the stack icon that's under the inventory boxes(When no item is selected, the icon is grey, when an item is selected, it's gold).
After you click it, it'll turn green, like this
- Press an open inventory box.
- A pop-up will show up asking you how many stacks you want to split, so type the number of how many of that item you want removed/split from the stack. Then click the blue "Split" button.
- After you've done this, the amount of the item you split will be moved to the empty inventory box.

Alternative:
If you're on desktop, you can hold ctrl and shift, then click the stack you want to split, click an empty inventory box, and then follow the same directions for the pop-up as above. ^^

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I hope this helps explain. ^-^

Whoops, it was right in front of me all along. Thanks!