If you are hired as a software engineer, yes, the systems you develop as part of your job belong to the company.
If someone hires you to cut the grass, and you create a machine that does it (the "Yardba"(tm)), there's no reason that machine would belong to the employer - creating it was not a part of your job.
No, this depends entirely on your employment contract. (Though I do agree that presumably a groundskeeper would not have intellectual property clauses in their employment contract) I've worked for two companies and they had different clauses; one said something to the effect that they owned any IP in their business interest (regardless of whether it was created on company time or company equipment); the other was narrower and said that they owned IP created outside of work only if it was dependent on company confidential information.
If someone hires you to cut the grass, and you create a machine that does it (the "Yardba"(tm)), there's no reason that machine would belong to the employer - creating it was not a part of your job.