How did the assassination of the archduke austria-hungary lead to war between russia and germany

The story went like this:

Austria blamed  a Serbian rebel and by extension Serbia and gave them an "ultimatum" ( to search for the culprit in their land) (frankly, it was an extreme ultimatum with Austria demanding power that could threaten Serbia's independence. )

Serbia found it too much and asked Russia (with whom it had an alliance) for help. Russia agreed.

And so Russia, Germany and half of Europe was at war.

Austria asked Germany for help- Germany agreed.