Salesforce: Roll up Activities to Account

In Lightning, activities can be configured as Activity Timeline or Related Lists. As an admin, you can configure this from Setup | User Interface | Record Page Settings | Default Activities View (desktop only).

However, users also able to personalize the activities view on their own as Activity Timeline or Related Lists from Settings | Display & Layout | Record Page Settings.

Another thing that admin can configure is to enable roll-up activities to a contact's primary account, this means activities (event, task, and email) links to Contact will appear in their direct Account too. To configure this, navigate to Setup | Feature Settings | Sales | Activity Settings, enable roll up activities to a contact's primary account.

Additionally, activities (event, task, and email) in Opportunity and child objects (with Master-Detail relationship to Account only) will appear in Account too, this behaviour is applicable for both views with Activity Timeline or Related Lists. However, Case activities will NOT roll-up to Account.

In summary,

* with "roll-up activities to a contact's primary account" is enabled

^ child of Master-Detail relationship

Notes: to enable Contacts links to multiple Account, looks for Account Settings in the Setup menu, then enable Allow users to relate a contact to multiple accounts.

Reference: 

• Enable Roll Up of Activities to a Contact’s Primary Account
• Activities View