If you have two Apple IDs and want to 'combine' them
First, it is not possible to actually combine or merge two Apple IDs. However, there is a solution to make them work together for you.
You can use Family Sharing to access subscriptions and purchases such as Apps, Music, Movies, etc. from either. If you have an Apple ID you use most often for things like messaging or email I recommend making that your primary Apple ID. I will be referring to the primary Apple ID as Apple ID (1).
From Apple ID (1) create a Family Group then invite your secondary Apple ID, Apple ID (2). From Apple ID (2) you would then accept the invite.
Invite people to join your family
If you have subscriptions under both Apple IDs you can share them between each. In this scenario you may be able to save money by canceling duplicate subscriptions from Apple ID (2) then sharing them from Apple ID (1). You may also find Apple One could save money depending on what subscriptions you use.
Add a family member to your shared subscriptions - Apple Support
For sharing your previously purchased content you would utilize Purchase Sharing. From Apple ID (1) set up the feature then download the content you want from Apple ID (2)s purchases.
How to share purchases with your family - Apple Support
How to download apps and content that your family members purchased - Apple Support
What types of content can I share with my family? - Apple Support
If you have photos or videos from Apple ID (2) that you want to be able to access from Apple ID (1) you can set up a Shared Photo Library.
How to use iCloud Shared Photo Library - Apple Support
If you have other content from Apple ID (2) such as "Data that you store with iCloud such as contacts, calendars, notes, bookmarks, reminders, email, photos, videos, and document." may be able download a copy of it from Apple's privacy tool to then import it to Apple ID (2).
Get a copy of the data associated with your Apple ID account - Apple Support