Hide allows you to become Hidden from creature(s) observing you.
Success If the creature could see you, you’re now hidden from it instead of observed. If you were hidden from or undetected by the creature, you retain that condition.
It further clarifies specific ways this Condition ends (in addition to the Condition itself)
You cease being hidden if you do anything except Hide, Sneak, or Step. If you attempt to Strike a creature, the creature remains flat-footed against that attack, and you then become observed
The stipulations in the Hidden Condition/Rules are easily applied, simply removing the stipulation that you lose it
While you're hidden from a creature, that creature knows the space you're in but can't tell precisely where you are. [...] A creature you're hidden from is flat-footed to you, and it must succeed at a DC 11 flat check when targeting you with an attack, spell, or other effect or it fails to affect you. Area effects aren't subject to this flat check.
A creature might be able to use the Seek action to try to observe you.
Hiding renders you Hidden from an enemy, even if they would otherwise have clear vision of you. This causes them to have to roll Seek (to remove the Hidden Condition) or roll flat checks for any targeted effect. They are aware of your location, but you are hard to 'pin down' to affect directly.
Sneak allows you to increase your Hidden condition to Undetected, meaning the enemy completely loses track of which space you're in. This is a little harder to adjudicate. From Legendary Sneak
You can Hide and Sneak even without cover or being concealed.
and Sneak allows
At the end of your movement, the GM rolls your Stealth check in secret and compares the result to the Perception DC of each creature you were hidden from or undetected by at the start of your movement. [...] You don’t get to roll against a creature if, at the end of your movement, you neither are concealed from it nor have cover or greater cover against it. You automatically become observed by such a creature.
Success You’re undetected by the creature during your movement and remain undetected by the creature at the end of it.
You must have the Hidden Condition (not cover or concealment) in order to attempt to Sneak, so Legendary Sneak does not help there. The example presented does not work, although it is still valid if said Legendary Sneaker had previously used Hide. You can become Undetected even if you don't end your movement behind cover or in concealment. The Undetected Condition/Rules make a little less sense in this context, but are at least clear about the result.
When you are undetected by a creature, that creature cannot see you at all, has no idea what space you occupy, and can't target you, though you still can be affected by abilities that target an area. When you're undetected by a creature, that creature is flat-footed to you.
A creature you're undetected by can guess which square you're in to try targeting you. It must pick a square and attempt an attack. This works like targeting a hidden creature (requiring a DC 11 flat check), but the flat check and attack roll are rolled in secret by the GM, who doesn't reveal whether the attack missed due to failing the flat check, failing the attack roll, or choosing the wrong square.
A creature can use the Seek action to try to find you.
If you have the Hidden Condition, Legendary Sneak allows you to stealth anywhere, causing enemies to lose track of you during your movement. The result is that you blend perfectly wherever you end up, and enemies don't know your location. You cannot be targeted directly unless the creature succeeds at a Seek action.