Alphabetical index entries are not created from paragraphs. They are an “annotation” anchored (or attached) to some location in your text. Initially, at Insert
>TOC & Index
>Index Entry
time, they can be built from the current selection or word.
Alternatively to your requirement, you can create an entry manually and tick Apply to all other occurrences checkbox to capture all uses of the same term in your document.
Another approach, avoiding the need to tag your document, is to use a concordance file containing the words to include in the index. Such a file can be created when you insert the index: tick Concordance file and the drop-down menu File allows you to New to create an in initial version. The concordance file is a plain text file you can also edit outside LO with a text editor.
PS: “latest stable version” does not mean anything for anybody else than you because update cycle does not proceed at same rate over all platforms and distros. E.g. yu’re under Mint 22.1 and this is probably different from Mint 24 or 25. Prefer to mention explicitly the release. I am under 24.8.5.2 – lastest stable in Fedora 41 – though 25.2.x.y has already been released as “stable”.