You should divide in known size chunks (e.x 4kb each) and knowning the total file size and the chunk size you can compute the percentage downloaded each time a chunk is finished.

function PercentageCalculatorFactory(totalFileSizeInBytes){

    var totalDownloaded = 0; 
    const chunkSize = 4096;
    
    return function onChunkDownload(){
    
        totalDownloaded ++;
    
        var sizeDownloaded = totalDownloaded * chunkSize; 
    
        return sizeDownloaded / totalFileSize * 100; 
    }

}


//usage 
var percCalculator = PercentageCalculatorFactory(10000); 

//get the file size using an api that serves some metadata about the file, in this case the size but the name might be needed etc.

async function testDownload()
{
    try{
        var urls = ['https://localhost:54373/analyzer/test1','https://localhost:54373/analyzer/test2']
        var fullFile = new Blob();
        for (let x = 0; x < urls.length; x++){
            await fetch(urls[x]).then((res) => {
                if (!res.ok || res.status !== 200) {
                    throw('Download failed');
                }
                var newPercentage = percCalculator(); 
                //do something with percentage 
                console.log( newPercetage ); 
    
                return res.blob();
            }).then(data => fullFile = new Blob([fullFile, data]));
        }
        let url = window.URL.createObjectURL(fullFile);
        let a = document.createElement('a');
        a.href = url;
        a.download = "WTF.ini";
        a.click();
    } 
    catch(e){console.log(e)};
}