How to merge accounts?

I have two accounts (personal one and work one) and I want to merge all activity from this last one into the first one. Then, I would remove that one and remain using this one.

What you want is described in detail here:
https://docs.github.com/en/github/setting-up-and-managing-your-github-user-account/managing-user-account-settings/merging-multiple-user-accounts

It is not a merging. My unique account does not inherit e.g. contribution activity from the other one.

That is pretty much how far the GitHub Docs go. As far as I’m aware there is no other way of merging.

What exactly are you looking to preserve? I think you can just remove the email addresses used in Git from account A and put them on account B to transfer any commit attribution.

Late. I already deleted one account and get ready all the process.