Generally speaking, no, you cannot reach into standard components and alter their behavior except in specific ways provided by Salesforce. For components, you'll find much of that configuration available to you upon selecting the component in the Lightning App Builder on your record page.
The Activity timeline offers very little customization. You will not be able to expand entries by default. Your modifications are largely limited to changing the fields displayed for each Activity line item; you can achieve this by editing the Task Compact Layout.
Beyond those allowed-for customizations, your next steps would be to install a third-party component from the AppExchange or build your own component.