you can use this to get all company names:
var AllCompanyNames = allData.SelectMany(u => u._rows.Select(t => t[5])).ToList();
and this, to get more property:
var Rows = allData.SelectMany(u =>
u._rows.Select(t => new
{
CompanyName = t[5],
Other1 = t[1],
Other2 = t[2]
}))
.ToList();
and this, if you need to check any condition:
var FilteredRows = allData.SelectMany(u =>
u._rows.Select(t => new
{
CompanyName = t[5],
Other1 = t[1],
Other2 = t[2]
}))
.Where(u => u.CompanyName == "XXX")
.ToList();