I just upgraded from CS6 2018 to 2020. Now whenever I select a white background and press delete, a fill popup shows up instead of making the image transparent. How do I change this?
Photoshop inserts background color to the deleted area in 2 situations:
1) Your image has color mode which hasn't transparency. RGB and CMYK have it. Set the mode to RGB by applying Image > Mode > RGB
2) You have only a background. Open the layers panel to see the situation. Remove the lock icon or double click the layer in the layers panel to get a possibility to convert the background to a layer which can have transparent areas
Photoshop opens JPGs as "background only". I bet most of the people who want to make edits to a JPG in Photoshop either duplicate the background to get a layer or convert it to a layer.
You can flatten a multilayer image to "background only" by applying Layers > Flatten image. By doing it you throw away all those editing and recomposing possibilities which layered structure provides.