rsa – To what extent are countermeasures against side channel attacks effective / used regularly by everyone?

Is it true that most of them do not offer absolute confidentiality, but rather reduce the risks?

No, many completely eliminate the risk. The use of temporal implementations of algorithms ensures that the secrets being processed have no effect on timing, thus making such auxiliary channel attacks impossible.

Are there any absolute solutions to existing attacks against secondary canals?

Yes, for some classes of auxiliary channel attacks such as timing based. See above.

Since these countermeasures are not widely known to the general public, is it true that people can not use these measures? Is it also true that these devices are usually not integrated with most consumer devices?

This is often the case, especially when the product does not use an established and popular library such as OpenSSL, which provides these mitigations, but instead attempts to design it on its own. These devices are often very vulnerable to side channel attacks, and the abuse of these weaknesses can be practical.