An ATM is a standing arrangement with a broker to sell up to a dollar amount of stock over time. There is no discount and no announcement on the day of each sale; the shares simply become part of normal volume. Sales are disclosed after the fact in the quarterly filings, so you usually learn the share count moved weeks later.
This is the standard financing tool of cash-burning small caps and a structural headwind behind many meme-stock rallies: every spike in price and volume is an opportunity for the company to fund itself. Check the filings for an active ATM before assuming a squeeze has no supply.
Example: a $500M ATM on a stock averaging $10 and 8M shares of daily volume. Selling 10% of volume means 800K shares, or $8M, a day. The full program takes about 63 trading days and adds 50M shares.
Related: dilution, shelf-registration, meme-stock, authorized-shares, float