Participants will receive atezolizumab in combination with trastuzumab, pertuzumab, and docetaxel every 3 weeks.

Participants will receive atezolizumab in combination with trastuzumab and pertuzumab every 3 weeks for 2 cycles, followed by docetaxel, carboplatin, trastuzumab and pertuzumab every 3 weeks for 6 cycles. Breast surgery will be performed no later than 6 weeks after neoadjuvant therapy. Upon the completion of surgery, participants will receive 12 cycles of single-agent trastuzumab every 3 weeks.

Participants will receive atezolizumab in combination with trastuzumab emtansine every 3 weeks for 2 cycles, followed by docetaxel, carboplatin, trastuzumab and pertuzumab every 3 weeks for 6 cycles. Breast surgery will be performed no later than 6 weeks after neoadjuvant therapy. Upon the completion of surgery, participants will receive 12 cycles of single-agent trastuzumab every 3 weeks.

Participants with HER2-positive metastatic breast cancer/unresectable locally advanced breast cancer who received prior treatment with trastuzumab and a taxane chemotherapy will receive atezolizumab in combination with trastuzumab emtansine at the dose determined from stage 1, every 3 weeks until disease progression, lack of clinical benefit, or unacceptable toxicity.

Participants with HER2-positive metastatic breast cancer recently progressed on an HP containing regimen will receive atezolimumab in combination with trastuzumab and pertuzumab every 3 weeks until disease progression, lack of clinical benefit, or unacceptable toxicity.

Carboplatin will be administered at an initial target of area under the curve (AUC) of 6 milligrams per milliliter*min (mg/mL*min) via an IV infusion on Day 1 of every 21-days for 6 cycles.

Docetaxel 75 mg/m^2 administered via IV infusion on Day 1 every 21-days for 6 cycles.

Pertuzumab 840 mg loading dose then 420 mg administered via IV infusion on Day 1 of every 21-day cycle.

Trastuzumab 8 milligram per kilogram (mg/kg) loading dose, then 6 mg/kg administered via IV infusion on Day 1 of every 21-day cycle.

Trastuzumab emtansine 3.6 mg/kg administered via IV infusion on Day 1 of every 21-day cycle. Dose de-escalation may occur for trastuzumab emtansine, from the full trastuzumab emtansine dose at 3.6 mg/kg on Day 1 of every 21-day cycle, potentially to 3.0 mg/kg (Cohort 1C) or 2.4 mg/kg q3w (Cohort 1D).

Doxorubicin will be administered at 60 mg/m^2 every 2 weeks as an IV bolus over 3 to 5 minutes or as an infusion over 15 to 30 minutes.

Cyclophosphamide will be administered at 600 mg/m^2 on Day 1 of each 21 day cycle as an IV bolus over 3 to 5 minutes or as an infusion, in accordance with local policy.