The encoders follow the published specs: Code 128 uses Start B, one symbol per ASCII character, a modulo-103 checksum and the Stop pattern; EAN-13 uses the official L/G/R code tables, guard bars and the standard check digit, which the tool validates as you type and fixes with one click when it is wrong. Scanners read the output like any other barcode. Whether it scans in the real world also depends on print size, contrast and the quiet zone — print at a sensible size, keep black bars on a light background, and never crop the margins.
Code 128 carries text, URLs and SKU codes of almost any length — the workhorse for labels and inventory; EAN-13 is numeric-only and fixed at 13 digits, as retail product codes require. Only these two symbologies are implemented — there is no QR, ITF, Code 39 or standalone UPC-E here. Code 128 Subset B covers ASCII 32–126, so emoji and accented characters are refused with a clear message instead of being silently mangled. Encoding and rendering happen 100% in your browser; your text is never uploaded.
Also useful: QR Code Generator.