Hey,
Following are the primary/most used means for creating a new person in Marketo
1. Manual Person Creation from Marketo > DB > New Person
2. Using Lead API
3. Web Form Fill-out
4. CRM
5. List Import
6. Munchkin API
7. Other Misc. custom/launchpoint API based services, etc.
Check out the Source Type constraint values in the Person was Created filter for the complete list of sources a person can be created in Marketo, ref. snapshot below:

Also, "Person Source" field is not populated automatically and you would have to set up smart campaigns based on your ingestion sources. However, for the form fills, you can also add Person Source field as a hidden field on the Marketo forms to capture the apt values. Let me know if this answers your question or in case if you need more details!