The bathroom generates two problems and both are mineral: limescale where water sits, and blockages where it drains. Almost everything else in here follows from those.
Limescale
It is calcium carbonate left behind as water evaporates, so it forms worst where water sits and dries: taps, shower heads, screens, the kettle. It needs acid, which means vinegar or a proper descaler, and it needs time rather than scrubbing.
- Shower head. Bag of vinegar tied round it, overnight.
- Taps. A cloth soaked in vinegar wrapped round the spout beats wiping.
- Screens. A squeegee after every shower does more than any product.
Blocked drains and toilets
Most sink and shower blockages are hair and soap, and the fix is mechanical rather than chemical: a drain snake or even a straightened coat hanger will pull out more than a bottle of anything. Chemical unblockers are a last resort and should never be mixed or stacked, see what not to mix.
For a blocked toilet, a plunger and patience work far more often than people expect. If the bowl is full, bail some out first so it does not go over.
Mould on sealant and grout
Bathroom mould is a ventilation problem wearing a cleaning problem's clothes. Clean it, then fix the airflow or it is back within a month. See heating, damp and energy. Silicone sealant that has gone black all the way through cannot be cleaned; it has to be cut out and replaced.