A Research Study to See How Much CagriSema Lowers Blood Sugar and Body Weight Compared to Placebo in People With Type 2 Diabetes Treated With Once-daily Basal Insulin With or Without Metformin

Inclusion Criteria: * Male or female (sex at birth). * Age 18 years or above at the time of signing the informed consent. * Diagnosed with type 2 diabetes mellitus ≥180 days before screening. * On stable once-daily dose of basal insulin (minimum of 0.25 units per kilogram per day (U/kg/day) or 20 U/day) alone or in combination with metformin (at effective or maximum tolerated dose as judged by the investigator) for 90 days prior to screening. * Glycated haemoglobin (HbA1c) 7.0-10.5 percent (53-91 millimoles per mole \[mmol/mol\]) (both inclusive) as determined by central laboratory at screening. * Body Mass Index (BMI) greater than or equal to 25 kilogram per square meter (kg/m\^2) at screening. BMI will be calculated in the electronic case report form (eCRF) based on height and body weight at screening. Exclusion Criteria: * Female who is pregnant, breast-feeding or intends to become pregnant or is of childbearing potential and not using a highly effective contraceptive method. * Renal impairment with estimated Glomerular Filtration Rate (eGFR) less than 30 milliliters per minute per 1.73 square meter (mL/min/1.73 m\^2) as determined by central laboratory at screening. * Treatment with any medication for the indication of diabetes or obesity other than stated in the inclusion criteria within 90 days before screening. * Uncontrolled and potentially unstable diabetic retinopathy or maculopathy. Verified by an eye examination performed within 90 days before screening or in the period between screening and randomisation. Pharmacological pupil-dilation is a requirement unless using a digital fundus photography camera specified for non-dilated examination. * Known hypoglycaemia unawareness as indicated by the investigator according to Clarke's questionnaire question 8. * Recurrent severe hypoglycaemic episodes within the last year as judged by the investigator.