See what's inside a Photoshop file without Photoshop: the flattened composite, rendered in your browser, saveable as PNG.
A PSD stores, alongside its layers, a flattened composite of the whole image, the 'Maximize Compatibility' copy Photoshop saves so other programs can preview the file. This viewer parses the PSD structure directly (header, then the composite's channel data, both uncompressed and PackBits-RLE compressed files) and renders that composite to a canvas, which you can then save as a PNG. For the overwhelmingly common need, 'a designer sent me a PSD and I just need to see it / use the final image', that is the whole job.
Scope, honestly: this reads the flattened composite only, no layer inspection, no text extraction, no editing, and supports the standard 8-bit RGB and grayscale documents. Files saved without compatibility mode have no composite and cannot be previewed this way; 16/32-bit and CMYK documents may render incorrectly or not at all. Nothing is uploaded: the parsing happens in the page, which also makes it safe for client work under NDA.
Why can I not see the layers? Layer-by-layer parsing with blend modes is a much larger undertaking, and the composite is what most people need. The EXIF/metadata-style tools on this site cover other inspection needs; full layer editing is Photoshop/Photopea territory.
The viewer says no composite found, why? The file was saved with Maximize Compatibility off, so Photoshop stored no flattened preview. Ask the sender to re-save with compatibility on, or open it in an editor once to add one.
Will the PNG match the PSD exactly? It matches the saved composite, which is the document's own flattened rendering, including any state the file was saved in (for example, hidden layers stay hidden).