Singing at a normal indoor volume is fine even in apartments. Maybe you can't belt music out at the top of your lungs but despite the lease agreement no city will allow the enforcement of a rule that no singing at all is allowed in apartments.
So I think singing should be fine legally speaking unless you live somewhere with a lot stricter limits, especially during the day. It's a good idea to check with neighbors though. Often they won't mind if you do it as long as you avoid certain times or certain rooms. That's how we (sort of) resolved things with our student friends... and thankfully we moved out of there pre-quarantine :)
That really depends on what kind of music you like to sing
Seriously though. How would singing be relaxing if you had to deal with angry neighbours every few times you sung?