Avyayibhava is another compound form. It is also called “uninflected form”. Avyayibhava compound emphasizes more on the first compound word rather than the second one.
अनु + रूप → अनुरूप → अनुरूपम्
anu + rūpa → anurūpa → anurūpam
After + form → “after-form,” according to a form → accordingly, naturally
Avyayibhava seems to emphasize the first member of the compound. Here, anurūpam is not a sort of rūpam; the word rūpa here just serves to qualify the nature of anu. For that reason, the first word is the main idea and the second is the qualifier. So we will put Avyayibhava in the qualifier part of the second word.
Tatpurusha compound with verb roots
The second word of a tatpurusha can be an ordinary verb root. Consider the compound analysis below:
भुजैर् गच्छतीति भुजगः
bhujair gacchatīti bhujagaḥ
“It goes with coils”, i.e. a snake