Best way to clean inside the car

There's no real amazing science to it, that marketing from manufactuers would lead you to believe.

APC is your friend for almost all of your interior jobs (APC = All Purpose Cleaner) - it's just a matter of using the right dilution.
A mild mixture of APC and water in a spray bottle, a few different sized brushes, and a few microfibre cloths and you'll do a good job.
Spray on the mixture, agitate with the brushes, wipe to a reasonably dry finish with the microfibre cloths...plastics, leather, etc. This will leave a nice matt finish on plastics and leather too; you may wish to condition the leather, but thats personal choice. I prefer the plastics and leather in my car untreated.
As for the car matts, if you can get a hold of a wet vac, great, but if not, same method with a stronger mixture and a stiffer brush. Leave them out to dry if you've no wet vac, as if you leave them in damp, the windows will constantly fog up until the dampness is gone. Dont put them in the washing machine - will ruin them.

Chemical Guys Fabric Clean is a great product for the likes of milk, yougart, etc, but an APC mix should do the job fine; something like Daisy from Tesco's or W5 from Lidl. Obviously all the car care manufactuers make their own form of APC too, which will do the job. Basically read the instructions and make a reasonable judgement about the dilution ratio you should use...if really unsure, be safe and start with a weak mixture and work your way up. All you might want then is a few squirts of a nice car air freshener!