You can use the <user>#send method as seen in the docs

If you want to send to the author of the message, use the <user>#send method or

message.author.send()

If you want to send to a user by id, fetch a user and send using the .send() function. Make sure they are in the cache. To fetch a user, use the <user>#fetch method.
Note: change client to what you defined client as.
Code:

const user = <client>.users.cache.fetch(id);
user.send();

Yes you can! If you have their ID, it is very easy.

const user = client.users.cache.fetch("<USER ID>");
user.send("Hello, world!");

If you don't have the user id, don't worry, you can still do it by username, as long as they are in a server the bot is in too.

const user = client.users.cache.find(user => user.username == "<USER NAME>");
user.send("Hello, worlD!");

please mark as correct answer, thanks!