The question concerns special signs from antiquity. In the special characters section we can find AE - Æ, AV - Ꜹ, AU - Ꜷ ligatures but I don’t find AR ligatures. Question: is the AR ligature in LibreOffice resources, possibly what is the matter of adding such characters in such a way that web browsers can read it, thank you for your help.
Writer handles the Unicode character repertoire. As far as I know, unusual ligatures like AV, AU or AR are not defined in Unicode. Consequently, if you already found AV, this is a specific extension in a font (you didn’t mention it either). Such “private” ligatures are managed by the font renderer which makes use of dedicated internal tables in the font file.
AE and OE are not defined as ligatures by Unicode, though they look like ligatures. AutoCorrect may convert a sequence of A E or O E (and their lowercase counterparts) into Æ or Œ but this is not compulsory (and may change meaning in some languages).
The only Latin Unicode ligatures are in the Alphabetic Presentation Forms block starting at U+FB00. They are ff, fi, fl, ffi, ffl, ft and st.
This does not mean that a font provider can’t add other special purpose ligatures, but they are then enabled through the font features subdialog in Writer.