In short: residence permits are renewed before they expire (with an allowed later window); while it is decided you can keep residing and working, and in renewals silence is usually positive.
In person or by representative? The source does not detail this: the application can usually be filed online or through a representative, but fingerprinting and TIE card collection are in person. By default, prepare for personal appearance and check with your Oficina de Extranjería.
When to apply: apply before expiry. Many permits are renewed in the 60 days before and an application is also accepted up to 90 days after expiry (filing late may carry a minor penalty). The exact windows vary by permit type.
While it is processed: if you filed on time, your permit's validity is extended until it is decided; you can keep working.
Administrative silence: in renewals it is usually positive — after 3 months with no decision, it is understood granted (unlike initial applications, where silence is usually negative). There are exceptions, so check your case.
Official source: Immigration Regulation (RD 1155/2024); per-type renewal sheets at inclusion.gob.es.
Informational only; deadlines and the meaning of silence vary by type — check the official source and a professional for your case.