AppleID accounts cannot be merged per-se. Any purchased Apps, media, subscriptions and Apple services are forever tied to the AppleID account from which they were purchased. This limitation can in part be overcome by using Apple’s Family Sharing.
The best that you can hope to achieve is to merge your files and data. To do so, simply sign-out from the AppleID that you use least (or is less important to you) - and continue to use the AppleID that you use most on all of your devices.
When signing-out of an AppleID, you have opportunity to save a local copy of any iCloud data associated with that AppleID account. If you choose to save a local copy while signing-out, this data will be merged with the AppleID account that you next use to sign-in to iCloud from that device.
If you do not save a local copy of iCloud data when signing-out, all associated iCloud data will be deleted from the device - and will not be merged with the AppleID that is next used when signing-in to iCloud.
As such, data (including Photos, Passwords, Notes, Contacts, Calendars, Reminders Notes, Contacts etc.) associated with one AppleID account can be merged with that of another - but perhaps be mindful that the merge cannot be undone - but would require unpicking through manual deletion.
