Hello there!

Covalent bonds are formed as a result of sigma bond or pie bond. In your question, you have mentioned four cases.

In the first case, the overlapping of dyz+dyz leads to a head on overall which forms a sigma bond

In the second, third and fourth case where py+dxy, py+py and py+dyz are respectively overlapped, leads to sidewise overlapping and forms pie bonds.

From the above theory, we can conclude that, all the four cases leads to the formation of covalent bonds.

Hope this answer resolves your query.