How to merge two Apple Accounts on my Devices?

I am having to post this separately because there is a limit to post length.

If you have important information in each account and you wish to merge them, Apple has no formal account merging or transferring process. The things below will help you in doing ongoing or one-time transfer of as much as possible from one account to the other, but some things can never be transferred (namely purchases), only shared.

You can use various iCloud sharing features for some ongoing data sharing.

Share folders with iCloud Drive --> Share files and folders in iCloud Drive on iCloud.com - Apple Support

Read How to use Shared Albums in Photos on your iPhone, iPad, and Mac - Apple Support about how to share photos.

Sharing music playlists —> How to share an Apple Music playlist on your iPhone or iPad - Apple Support

Apps and media bought with one Apple account are permanently associated with that account. Look into using Family Sharing to continue to be able to use items purchased with one Apple account with other Apple accounts, and for some data sharing features.

- Family Sharing - What is Family Sharing? - Apple Support

- Set up Family Sharing - Set up Family Sharing - Apple Support

The next section may sound tricky but it lets you merge some things on a device where you have already signed in with an old account but want to use a new account and incorporate data that is already on the device from the old account. Incidentally, the data will then propagate to other devices using iCloud with that new account. (I haven't tried this with a computer but it may also work with a computer account signed into an old account.)

If you have signed into the first Apple account on a device then fortunately when you sign out of that account it asks you if you wish to retain the data from the old account on your device (toggle the ones you want to on). If you keep data on the device then when you sign in with the other account it will merge this data with the account signing into the device. You can later delete individual items from the old account that you do not want. Similar information can be found in this post: https://discussions.apple.com/thread/255224741?answerId=259726272022#259726272022 , and a step-by-step in this User Tip: https://discussions.apple.com/docs/DOC-250008248 . Note that only content physically present on the device such as contacts, photos and videos will be retained. Meta data such as a game play scores that are tied to a specific account or are only on a vendor's Apple Account-specific database remotely will not be retained. Also turn off storage optimization before doing this. With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. Make sure photo optimization is turned off so the full quality versions from the old account are actually on the device before merging the content into your new account. Manage your photo and video storage - Apple Support You can turn optimization back on again once you are fully using your new account on the device.

Even though apps may remain on a device and may work for a while, you will not be able to update them or redownload them unless you buy them again with this account or download them with this account using Family Sharing.

Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your devices - Apple Support

If this involves a device changing ownership, the person should also change the ID set in Find My. Turn off Find My on your devices and items - Apple Support

If another Apple account is signed in you may need to enter the password for that account to sign out of that account first.

Click on this link to learn how to 'Sign in with your Apple ID' --> https://support.apple.com/HT204053

I have the feeling that this is going to turn into a horribly long reply.

You are getting that "taken" message because it sounds like you are trying to create yet more accounts. Stop making more accounts.

The easiest solution is to abandon one account and use the other one on all devices. To use all accounts on the devices, sign out of the existing account if a device is not using the one you wish to keep.

Read this document to learn how to sign out of an account on your iPhone, iPad, iPod touch, Apple TV, or Mac --> Sign out of iCloud on your devices - Apple Support

For an Apple mobile device:

1. Go to Settings, then tap your name. 

2. Scroll down to the bottom.

3. Tap Sign Out.

4. Enter your Apple ID password and tap Turn Off. (Read this if you If you forgot your Apple Account password - Apple Support ) 

5. Turn on the data that you want to keep a copy of on your device.

6. Tap Sign Out.

7. Tap Sign Out again to confirm that you want to sign out of iCloud.

Also of possible relevance:

Turn off Find My on your devices and items - Apple Support

Read this support article to learn how to sign in with an Apple Account on various kinds of devices --> Sign in to your Apple Account - Apple Support

For an Apple Mobile device:

1. Open the Settings app.

2. Tap Sign in to your [device].

3. Enter your Apple ID and password.

4. If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in.

If another Apple Account is signed in you may need to enter the password for that account to sign out of that account first.

Note that we now get confusing terminology. There can be multiple user accounts on a computer. Each user account can have either the same Apple Account signed into it, or it can have a different one. So when you start including computers what you need to do depends upon how you wish to configure the computer.

Not really following. You would need to log out of the first Apple ID and into the second. If you’re trying to add one of the two as an alias, you cannot.

But when I try to change the unwanted one and inserting the one I need, it tells me that it’s taken and doesn’t realize that I’m the same owner of the email, even though I have the same phone number as well as the same iCloud email registered on both ID’s.

I only want to use one Apple ID

I just want to have one Apple ID, gmail account on all of my devices. So I have to log out from the one that I want to keep and then insert that ID into the two other devices as a new id?

What if I decided to built my info@x.net email account and switch all of my devices to that particular Apple ID

Use an Apple iCloud email and things will go smoother. I had Gmail as well but switched