People arrive at this decision knowing they want a child and not much else about how. The three common routes get talked about as if they were variations on one idea. They are not. They differ in law, in money, in medical oversight, and in who the child can find when they are eighteen.
A known donor
Someone you meet, agree with, and can name. The child grows up able to know who they are — often able to meet them. This is the route with the most human contact and the least institutional protection: no clinic checks anyone, and whatever you agree between you is only as strong as the law in your country allows it to be.
It is also the route where the greatest number of assumptions go unspoken. A donor who imagines occasional contact and a parent who imagines none have not disagreed yet. They will.
A clinic donor
Screening, legal separation, and a process designed so that nobody has to negotiate. You give up choice — you will not meet this person — and you buy certainty in exchange. Clinics carry costs, waiting lists, and in some countries eligibility rules about who they will treat.
What clinics do well is exactly what private arrangements do badly: infectious and genetic screening, documented consent, and a legal position that has been tested before.
A co-parent
Not a donor at all. A second parent, raising the child with you, without the two of you being a couple. This is the largest commitment of the three and the one people underestimate, because it is a relationship with someone you are not in love with that has to last twenty years.
Choosing between them
The question that separates them is not “which is easiest”. It is: what do you want this child to be able to find out, and from whom? Everything else follows from that answer — the legal work, the money, the conversations.
Whichever route you take, take advice in your own country before anything is agreed. What is settled in one European country is contested in the next.