A phone is better for this than a laptop, because you can sign with a finger instead of drawing with a mouse.
A finger on glass is closer to a pen on paper than a mouse will ever be. Signatures drawn with a mouse look like they were produced with a mouse, which is not the impression most people want on a contract.
Expect to redo it. First attempts are usually too large and too shaky. Signing slightly slower than you would on paper helps, as does drawing larger and letting the tool scale it down.
If you sign documents often, cut your signature out once as a transparent PNG and reuse it. It looks identical every time, which reads as more professional than a fresh finger drawing on each document.
In many places an electronic signature is valid for ordinary agreements, but it depends on the document and your jurisdiction. Some documents require witnessing or notarisation.
Yes. Save it as a transparent PNG and place that on future documents.
No. The document and your signature both stay on the phone.