Linking internally to parts of a chapter?

You are not the first one to ask for it.
Yes, some other apps have that.

Here is how I do it if you are interested, and I suppose that you could leave them markers in there as a way to speed up the post-process in Sigil, and remove them once done (still in Sigil).

Check this and the previous post from me in that same thread :

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Another way of handling it :