Arma 3 and its Apex expansion are not officially available as a free download for the full version. The game is a and typically requires a purchase.
NVIDIA GeForce GTX 660 / AMD Radeon HD 7850 with 1 GB VRAM Storage: 50 GB available space Arma 3 on Steam download-arma-3-apex-pc-game-full-version-free
Sites like Instant Gaming or G2A often sell Steam keys for the Apex edition at discounted rates. System Requirements for Arma 3 Apex Arma 3 and its Apex expansion are not
However, there are legitimate ways to access some of its content or purchase it at a significantly reduced price: Official Ways to Play for Free (Limited) download-arma-3-apex-pc-game-full-version-free