Does someone who is retired and/or on disability still have to file taxes? if so, how?

If you are not required to file a tax return because your income is below the filing thresholds, you do not need to file a tax return but will still receive your stimulus payment.

Below is an extract of this IRS document:

Economic Impact Payments: What you need to know

Updated April 1, 2020

The distribution of economic impact payments will begin in the next three weeks and will be distributed automatically, with no action required for most people. Social Security beneficiaries who are not typically required to file tax returns will not need to file to receive a payment. Instead, payments will be automatically deposited into their bank accounts. However, some taxpayers who typically do not file returns will need to submit a simple tax return to receive the stimulus payment. When more specific details become available, we will update the coronavirus page.