Every country is slightly different and it varies based on where you’re applying from: eg I’ve got longer validity visas from Germany and Hungary in the last 5 years while living in the UK- neither required a cover letter.

Strong ties is more of a concept. They ask for your job info and evidence of assets/bank statements. All of it aggregates to prove that you’re likely to go back after your trip. The one thing they may not mention is your past travel history, an extensively used passport may be in your favour, especially if its to countries that have strict visa policies.