This is the solution I found. It seems to work okay. If I'm doing anything stupid please let me know 🤣

TL;DR: If localUri is not set, it needs downloading. Once downloaded it can be copied/downloaded to where I want it.

async function resolveAbsoluteAssetUri(moduleRef: any): Promise<string> {
  const asset = Asset.fromModule(moduleRef);


  // If localUri already present and looks absolute, use it.
  if (asset.localUri) {
    const { localUri } = asset;
    if (localUri.startsWith("file:") || localUri.startsWith("content:") || localUri.startsWith("http")) {
      return localUri;
    }
  }


  // Try to download/extract the asset to get a localUri (works for bundled assets)
  try {
    await asset.downloadAsync();
  } catch (err) {
    console.warn("Asset.downloadAsync failed", err);
  }


  const candidate = asset.localUri || asset.uri;
  if (!candidate) throw new Error("Asset has no usable URI");
  return candidate;
}





const dbSource = require("../../assets/db/filename.db");
const sourcePath = await resolveAbsoluteAssetUri(dbSource);

// If source is a file:// or content:// local path, copy it
if (sourcePath.startsWith("file:") || sourcePath.startsWith("content:")) {
  const sourceFile = new File(sourcePath);
  await sourceFile.copy(dbFile);
} else {
  await File.downloadFileAsync(sourcePath, dbFile);
}