Identifying wood type of broken stair treads and installing new risers
The wood looks like and probably is fir or pine. This is typical in many basements and transitional areas from house to garage for example where only a short stair is needed. It's hard to tell from the pic how long those stairs are.
In any event, if you decide to replace them there are code requirements that need to be met. Although they vary somewhat by jurisdiction below is a typical configuration that I believe will meet code in most areas - but you want to check locally before you build.
If you decide to repair the steps and want to add risers you do need to make sure that you don't compromise tread depth/run to less than that required.
Additionally, you will want to maintain 1&quot; nosing on the stairs. You can use fir for the risers or 1/2&quot; plywood is also suitable.