The inquiry into the handling of the pandemic has asked to see Boris Johnson's WhatsApp messages from when he was prime minister.
Hugo Keith KC, counsel for the inquiry, revealed that thousands of documents had been asked for.
"We have sought agendas, minutes and other documents associated with the core decision-making forum such as Cabinet meetings, Cobra meetings and ministerial implementation groups," he said.
"We've asked for ministerial submissions, Number 10 daily briefing documents, records of written and oral advice to ministers and details of internal communications including a WhatsApp group, which included the prime minister, Number 10 and other senior officials."
The inquiry has requested evidence for its second part, which will scrutinise political decisions and actions in relation to the pandemic between early January 2020 and February 2022, when the remaining COVID restrictions were lifted.
The Cabinet Office, Foreign Office, the Department of Health and Social Care, the Office of the Chief Medical Officer and the Scientific Advisory Group for Emergencies (Sage) are among the departments and groups that have been asked to provide evidence.
Initial responses from government departments said tens of millions of documents could potentially be relevant to the overall theme of module two.
Reviews of documents in the Cabinet Office alone could take more than three years, it said.
Mr Keith said the inquiry would take a "targeted approach", seeking documents relevant to the key narrative events and decisions covered by the second module.
Earlier, as he opened the second stage of the statutory inquiry, Mr Keith said the hearings would examine whether lives could have been saved by earlier lockdowns.
In his opening address, Mr Keith said the crisis placed "extraordinary levels of strain" on health, care, financial and educational systems and businesses, on top of individual bereavements.
Baroness Heather Hallett, the chairwoman of the inquiry, will examine the effectiveness of mandatory lockdowns in controlling the spread of coronavirus.
This will include "the relationship between the timeliness and the length of the lockdown, and the trajectory of the disease", Mr Keith said.
He continued: "How were economic and societal impacts, including the impact on physical health, healthcare provision, mental health, education and societal wellbeing, assessed and weighed in the balance?
"And perhaps, my lady, the single most important question: is it possible to say what the likely effects of earlier or different decisions to intervene would have been? The counterfactual proposition.
"Bluntly, would lives have been saved if the lockdowns had been imposed earlier or differently?"