Learn Swedish: Insidan blir polerad nu. - The inside is being polished now.

The verb blir creates a dynamic passive, meaning it describes an action or transition that is currently happening. The word nu (now) emphasizes this ongoing process. If you used är polerad (a stative passive), it would mean the action is already finished and you are describing the resulting state: 'The inside is polished'.

Could I use the s-passive and say Insidan poleras nu instead?

Yes, poleras is perfectly fine here and means the exact same thing. Both the s-passive and the bli-passive describe actions. The bli-passive is often chosen when you want to put a slightly heavier emphasis on the physical process of change, but in everyday speech, they are highly interchangeable in this context.

Why is the participle spelled polerad and not polerat?

In both bli and vara passives, the past participle acts like an adjective and must agree with the subject in gender and number. Because insidan is a singular en-word, the participle takes the matching form polerad. If the subject were an ett-word, like golvet (the floor), it would be golvet blir polerat.