Last date for filing revised and belated ITR for financial year 2018-19 is extended upto 31st July 2020.
Due Date for filing ITR for financial year 2019-20 is extended upto 30th November 2020.
Date of completion of investment for claiming 80C deduction in FY 2019-20 is also extended upto 31st July 2020 Hence you can do your investment upto 31st July 2019 and claim the same as deduction in your ITR for FY 2019-20.
Due date for furnishing form-16 for FY 2019-20 by employer to employee is extended to 15th August 2020. Hence please wait till due date and your employer may provide from 16 soon. You may contact finance department of your company for more information about this. 
Form 26AS can be downloaded anytime from the website of income tax department. But for complete form for a financial year can be downloaded only after last date of filing TDS return for march month of last year, so that all the figures are updated by all the deductors. For march-2020, last date for filing TDS return has been extended to 31st July 2020. Hence for complete data of FY 2019-20, you should download it after 31st July 2020.
For official announcement of extension of above dates, please click Here