Yep, more tummy time will build arm strength and core muscle strength. Sitting in a Bumbo seat or an Exersaucer/Jumperoo type toy will also help build core strength.
And remember that all kids are slightly different. The main strength thing that most kids this age can do is rolling over. About half the kids this age are learning to sit without support - that means that half aren't. And a few kids this age are beginning to crawl - but that means most aren't yet.