Once a month or two I run my washing machine on a hit wash with just a couple of old towels in there to 'clean' the washer. You will be amazed how soapy the water is. Running a hot wash like this gets rid of the detergent that is trapped in the machine and removes smells.

I just run every single load with a single "extra rinse" cycle, since I hate leftover detergent/smell on my clothes (and I already use "fragrance-free").

With the extra rinse, I've never had any kind of buildup of detergent in the machine. I know this because I wash my shower curtains with just water and bleach every ~month to remove mildew, and there are zero suds.

I have hard water so washing machine and dishwasher get limescale. I just run them on a hot cycle with a bottle of white vinegar every month or so. Leaves them sparkling clean. Same with the kettle. No need for nasty descaling products.

Many machines have a cleaning program to do exactly that. Machines without it should be avoided IMO.