Finding traps or secret doors requires a Wisdom (Perception) check (almost always), and disarming or unlocking them requires Thieves' Tools and a Dexterity (Thieves' Tools) check.
There has been a lot of discussion online about this, with many people offering many answers that mostly boil down to "do what you want but here's what I do."
It all starts with sort of ambiguous rules in the Player's Handbook, but this is the most relevant passage, found in the sidebar under Perception (DDB Basic Rules), "When your character searches for a hidden object such as a secret door or a trap, the DM typically asks you to make a Wisdom (Perception) check." The key word there is typically, but yes, Perception is the skill.
In a rare case, especially if we are role-playing the event, if there is something that may help a character deduce the presence of a trap, such as a wire sticking out, or a pool of blood under the door, or an electrical arc on the door handle, I might allow the player to choose to make an Intelligence (Investigation) check instead, if they want.
Of course, if Gary just got disintegrated by the skull motif on the door, you don't have to roll to figure out there's a trap. You might want to roll to see if there's a second trap, I guess, but that'd be a dick move by the DM, no?
Disarming a trap, or unlocking a door, requires specialized tools. Thieves' Tools. If you have some, you can make a Dexterity (Thieves' Tools) check to disarm or unlock. If you are proficient with those Tools, you can add your proficiency bonus. If you are not, you do not. If you don't have Thieves' Tools, you cannot make a Dexterity check to disarm or unlock the door.
But do you really need Thieves' Tools? Can you just make a straight up Dexterity check? After all, the RAW on Thieves' Tools (DDB Basic Rules) is "This set of tools includes a small file, a set of lock picks, a small mirror mounted on a metal handle, a set of narrow-bladed scissors, and a pair of pliers. Proficiency with these tools lets you add your proficiency bonus to any ability checks you make to disarm traps or open locks." Is the implication there that you can make an ability check without them? In short, no.
"Thieves' Tools" is a game construct, and what they look like could vary across cultures, players, characters, or campaigns. Whatever. But you can't disarm a trap with your shortsword, a crowbar, or that stick you picked up. They're not that expensive, buy some!
A lot of people seem to believe that an Intelligence (Investigation) check is used to find and/or disarm traps/doors. Nope.
In RAW, Investigation (DDB Basic Rules) is described as, "When you look around for clues and make deductions based on those clues, you make an Intelligence (Investigation) check. You might deduce the location of a hidden object, discern from the appearance of a wound what kind of weapon dealt it, or determine the weakest point in a tunnel that could cause it to collapse. Poring through ancient scrolls in search of a hidden fragment of knowledge might also call for an Intelligence (Investigation) check."
There's only one bit in there that might apply, "deduce the location of a hidden object." But I argue that you need clues do make a deduction. Hence the bit I note above under Perception to Find where I might let someone use Investigation instead of Perception is I think it is warranted.
If I am your DM, my house rule on Passive Perception is relevant to this discussion. In short, your Passive Perception will not find a secret door for you, but if high enough, might clue you in that you should make an active Wisdom (Perception) check. Read more here.
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