Can someone hack/access your phone from a different device without you knowing?

You've really given no indication that there is anything amiss. Not everything that we don't understand on our phones is the result of people doing something bad. In fact, it rarely is. Do not jailbreak your phone and make sure you have Two-Factor Authentication enabled to protect your Apple ID.

People will send you strange things through email (or text). All that means is that people are sending you strange things. Unless you click on a link, go to a website and give them information, you should be fine.

Glitches will happen in third-party apps. If they keep happening, try deleting the app and re-installing it. If that fails, contact the app developer. If a social media service such as Instagram offers ways to protect your account beyond just a password, use them. Keep in mind that your biggest danger with Instagram or Facebook is that now Meta knows everything about you. They are a much greater threat to you than hackers.

bleepnb022 wrote:

Are they able to text from your phone number? & HOW?!

There are websites out there that will allow you to "spoof" the sender of a text message. In other words, they make it appear it has come from someone else's number. This doesn't require any access to your phone or even that your Apple ID be compromised.

No, no one can get access to your apps unless they have had access to your phone (or unless they're willing to pay about a half a million dollars for your data). The routing of calls and SMS messages is handled by your carrier. While I supposed they could get hacked, that's many orders of magnitude beyond what we seem to be talking about.

Yes, it's possible to send an SMS to an email address. Pretty much no one bothers with that these days. But it's not indicative of anything nefarious.

No one can do anything with your phone unless you let them use it for an hour or two and you also give them your Apple ID and Password.

Have you looked at Apple's Safety Check for your iPhone?

How Safety Check on iPhone works to keep you safe - Apple Support

There are websites, I have been told, that allow people to do this. How it works, I couldn't say. I'm not a programmer.

There's really nothing you can do to stop it. You could change your phone number, I guess. That would be very annoying and if someone found it out the new phone number, they could do it again.

You can't stop someone from trying to spoof your phone number unless you know who they are.

Is that really the issue that you are having? People are responding to you because they received a "text" from you that you did not send?

Did your daughter's phone have a passcode on it? If so, it just got wedged in the couch cushions. That happens. Especially if there are lots of children around.

If she doesn't have a passcode on it, why not? Have her put one on it immediately. But even if she didn't, that still wouldn't give anyone access to your phone.

It's really not clear to me what caused you to start thinking that someone has access to your information. Again, people sending you strange emails or texts is pretty much normal these days. Spam is the scourge of our time. But, as long as you don't interact with it, it's generally harmless.

You can certainly restore all of your devices to factory state, not from a backup. That would remove everything on the phones. You could change your email address and your phone numbers. You can get a new Apple ID. However, doing all of that would be a major undertaking and would probably result in the loss of some data and access to purchased content. Probably for absolutely no reason.

bleepnb022 wrote:

Also, at the beginning of this relationship I got a spam text with a girl's picture. It was completely abnormal to receive something like this. I never responded but when he saw it, he took it upon himself to respond for me. Could this trigger something?

It was a spam text. It means nothing other than you got a spam text. Or, a wrong number. I got a very nice Father's Day greeting last week. But, I'm not a father. Someone made a mistake. I respond that they had a wrong number. They apologized. Responding to them wasn't dangerous. I also got a text yesterday telling me that I had a package that was being held for delivery until I provided more information. I reported that as junk and deleted it. That only would have been dangerous had I clicked on the link and given them the information they wanted (probably a credit card).

If you do not trust your partner, don't give them the passcode to your phone.

kae227 wrote:

Is it possible for my iPad, which doesn't have any sensitive data stored on it, to be hacked through WhatsApp, and if yes... will the hacked information only be accessible on the device being used?

No and no.