Here's a rough outline of the xml for a 2024 class.
<class>
<id-00001>
<ability type="string">Strength</ability>
<featurechoices />
<features>
<id-00001>
<level type="number">1</level>
<name type="string">My Feature</name>
<text type="formattedtext">
<p>Stuff Happens</p>
</text>
</id-00001>
</features>
<hp>
<hitdice>
<text type="string">d8 per My Class level</text>
</hitdice>
</hp>
<multiclassproficiencies>
<armor />
<skills />
<tools />
<weapons />
</multiclassproficiencies>
<name type="string">My Class</name>
<proficiencies>
<armor>
<text type="string">Light</text>
</armor>
<savingthrows>
<text type="string">Strength and Constitution</text>
</savingthrows>
<skills>
<text type="string">Choose 2: Animal Handling, Athletics, Intimidation, Nature</text>
</skills>
<tools />
<weapons>
<text type="string">Simple</text>
</weapons>
</proficiencies>
<startingequipment type="string">Choose %s: (A) 50 GP</startingequipment>
<startingequipmentlist>
<id-00001>
<items />
<wealth type="number">50</wealth>
</id-00001>
</startingequipmentlist>
<text type="formattedtext">
<p>Description</p>
</text>
<version type="string">2024</version>
</id-00001>
</class>
What I'd suggest (if you aren't doing this already) is create a class inside of FGVTT and then look at the db.xml file inside the campaign folder to see what the structure is (this is what I did for the above). This way you should be able to figure out what the structure is. (It's also just easier to create the class inside of FGVTT to be honest rather than trying to do the xml but I understand the desire to learn the coding).
One other thing to note is that the effects which relate to certain abilities are hard coded into the ruleset and don't appear in the xml code. For example if you add a Barbarian class to the character sheet then the Rage ability will appear in the abilities tab; and the effects coding will appear in the actions tab. However if this were a homebrewed class then nothing would be added to the actions tab.
EDIT: I see you got some answers elsewhere.