You question deserves a more thorough explanation of what you’ve already done and how. To do that, click the edit link (near the slanted pencil) under your question.
Meanwhile, here are general information about the TOC mechanism in LO Writer.
Your document (i.e. what you typed and how you formatted it) will be unaffected by TOC generation.
When you Insert>TOC & Index>Toc & Index, LO Writer scans your whole document from start to end to collect paragraphs styles Heading n. This means you gave this style to your headings, otherwise they will not be recognised as headings.
The heading paragraphs are then slightly transformed according to the template(s) defined in the Entries tab of the insert-TOC dialog. The template tells whether the heading numbering, the heading itself or the page number should be displayed and in which order. These elements may be separated by spaces or tabs and in this latter case if you want leading lines.
To allow maximum flexibility in appearance, TOC paragraphs are not styled Heading n but Contents n (with corresponding n). Consequently, if you want the headings in the TOC to be exactly formatted as the heading in the text (which is not a good idea because you usually have the page number in addition in the TOC), you only need to make Contents n identical to Heading n.
From this description, you see that having a different layout in the TOC is easy because headings and TOC are made independent by use of different styles and the template factory.
Adding a TOC has no back-fire effect on your document. It only adds a new portion of content which you can control without the fear of badly impacting the existing text.
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EDIT 2019-2-1
TOC collection
It is possible to include non-Heading n paragraphs in the TOC. Open the paragraph styles definitions and go to Outline & Numbering tab. The drop-down menu Outline sets the heading level. If this heading is to be numbered, associate the style to a numbering style (improperly named list style in the documentation) with drop-down menu Numbering style. This will allow to number/bullet the heading and also is the clue for Writer to capture the right numbering.
After that, the modified paragraph styles have the same properties as the built-in Heading n styles.
TOC formatting
There is no Contents-named table. Forgive me if my answer above was ambiguous.
The TOC is created as soon as you request it with Insert>… Writer then changes the original styles (Heading n and your modified custom styles) of the paragraph to Contents n to allow you to format TOC entries independently from main text.
If you already have designed a custom style for level-n TOC entries, you can substitute this custom style for built-in Contents n in the Styles tab of Insert>TOC & Indexes>TOC & Indexes dialog. But it is usually easier to modify Contents n.
