The chroma keyer makes a uniform-coloured background transparent, classically green, so whatever sits on the lanes beneath becomes the new background.
Open the chroma key editor Select the green-screen clip and open Effects → Chroma key. The full-screen editor shows the live composite while you tune.
Pick the key colour The default targets standard green. For anything else, use the colour picker to sample the backdrop from the frame.
Tune similarity Raise similarity until the backdrop disappears, then back off if the subject starts to erode. Edge blend softens the boundary.
Kill the spill Spill suppression removes the green cast that reflects onto hair and shoulders. A small amount is usually enough.
Add the new background Drop any clip, image or AI-generated scene on the lane beneath the keyed clip. With nothing beneath, the project background color shows.
Tips
Even lighting on the screen matters more than the screen itself: wrinkles and shadows are what break keys.
The preview keys the same way the export does, so what you tune is what renders.
For non-uniform backgrounds, use the eraser instead, or key what you can and erase the rest.