Unterstanding `Filesystem.readFile(...)` on mobile and web?

The doc of Filesystem.readFile(...) state:

Prop Type Description
data string | Blob The representation of the data contained in the file Note: Blob is only available on Web. On native, the data is returned as a string.

and

Prop Type Description
path string The path of the file to read
directory Directory The Directory to read the file from
encoding Encoding The encoding to read the file in, if not provided, data is read as binary and returned as base64 encoded. Pass Encoding.UTF8 to read data as string

But what does that mean? I get a base64 String as default on mobile. Will I always get a Blob on web? When I use Encoding.UTF8, will I get a String in base64 (instead of a Blob) or a real text String?

Main question: How can I get a base64 String instead of Blob on web to use the same atob() to decode my binary data?