Investigator-assessed Progression-free Survival (PFS) According to Response Evaluation Criteria in Solid Tumors Version 1.1 (RECIST v1.1)

From randomization to the first occurrence of disease progression (PD) or death from any cause, whichever occurs first (up to approximately 21 months)

Overall Survival (OS)

From randomization to death from any cause (up to approximately 36 months)

Investigator-assessed Confirmed Objective Response Rate (ORR) According to RECIST v1.1

From randomization up to approximately 21 months

Investigator-assessed Duration of Response (DOR) According to RECIST v1.1

From the first occurrence of a documented confirmed objective response to the first occurrence of PD or death from any cause, whichever occurs first (up to approximately 21 months)

Investigator-assessed PFS Rate According to RECIST v1.1 at 6 and 12 Months

Month 6, Month 12

OS Rate at 1 and 2 Years

Year 1, Year 2

Investigator-assessed PFS According to HCC mRECIST

From randomization to the first occurrence of PD or death from any cause, whichever occurs first (up to approximately 21 months)

Investigator-assessed Confirmed ORR According to HCC mRECIST

From randomization up to approximately 21 months

Investigator-assessed DOR According to HCC mRECIST

From the first occurrence of a documented confirmed objective response to the first occurrence of PD or death from any cause, whichever occurs first (up to approximately 21 months)

Time to Confirmed Deterioration (TTCD) Assessed Using European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire for Cancer 30 (EORTC QLQ-C30) Subscales

The following subscales of the EORTC QLQ-C30 will be used for the assessment: global health status/quality-of-life (GHS/QoL), physical functioning and role functioning. GHS and QoL are scored on a 7-point scale: 1=Very poor to 7=Excellent. Functioning items are scored on a 4-point scale: 1=Not at all to 4=Very much, with a higher score indicating a worse outcome. Scores will be linearly transformed with a minimum score of 0 and maximum score of 100. A higher score indicates a better outcome.

Up to approximately 21 months

Change from Baseline in GHS/QoL, Physical Functioning, and Role Functioning Assessed Using the EORTC QLQ-C30

GHS and QoL are scored on a 7-point scale: 1=Very poor to 7=Excellent. Functioning items are scored on a 4-point scale: 1=Not at all to 4=Very much, with a higher score indicating a worse outcome. Scores will be linearly transformed with a minimum score of 0 and maximum score of 100. A higher score indicates a better outcome.

Up to approximately 21 months

Percentage of Participants With Adverse Events (AEs)

Up to approximately 36 months

Serum Concentrations of Atezolizumab

Prior to the first infusion and 30 minutes after atezolizumab infusion on Day 1 of Cycle 1 (cycle = 21 days), prior to infusion on Day 1 of Cycles 2, 3, 4, 8, 12, 16 and at treatment discontinuation visit (up to approximately 21 months)

Serum Concentrations of Tiragolumab

Prior to the first infusion and 30 minutes after tiragolumab infusion on Day 1 of Cycle 1 (cycle = 21 days), prior to infusion on Day 1 of Cycles 2, 3, 4, 8, 12, 16 and at treatment discontinuation visit (up to approximately 21 months)

Percentage of Participants With Anti-drug Antibodies (ADAs) to Tiragolumab

Prior to the first infusion on Day 1 of Cycles (cycle = 21 days) 1, 2, 3, 4, 8, 12, 16 and at treatment discontinuation visit (up to approximately 21 months)

Percentage of Participants With ADAs to Atezolizumab

Prior to the first infusion on Day 1 of Cycles (cycle = 21 days) 1, 2, 3, 4, 8, 12, 16 and at treatment discontinuation visit (up to approximately 21 months)