You don't. The currently selected window or in this case Excel, becomes front most. Just the way the operating system works. Only windows created via application development can have their window status set in a manner to float above all other windows.
Single-click that source image that has the data you want to copy and tap the space bar. Now you have a Quick Look window open where you have the ability to click once on it to activate Live Text. Then you drag to select the text content on that image. If that is a screen shot of tabular data, select those cells, copy to the clipboard, and then with Excel open and the first workbook cell selected, paste and match formatting into the workbook.
The previous paragraph worked just fine for screen captured table data from a Numbers spreadsheet and the paste worked perfectly into Excel v16.100 on macOS Sequoia v15.6.