Xinyan is a Pyro elemental support/tank type character that uses claymore, similar to Noelle. Her elemental skill releases an area attack that creates a shield based on her defense, and gets stronger the more enemies she hits. If it hits at least 3 or more, the shield will also deal Pyro damage to nearby enemies. Her elemental burst deals a big physical attack, with a several Pyro attacks shortly after.
If you want her to be your main DPS, she works best as a physical damage dealer, instead of a Pyro damage dealer. This is because her elemental skill has a fairly long cooldown, the level 3 shield has a situational condition, and her big attack from her elemental burst deals physical damage. Her 4th ascension passive also increases physical damage. This means that for artifacts, you'll want to go with either 4 piece Gladiators Finale, or 2 piece Gladiator with 2 piece Bloodstained Chivalry. Ideally physical damage bonus on the cup, with crit rate on the crown, which can be good if you have her at constellation 1. If you have her at constellation level 2, where she gets 100% crit chance on her ultimate, then you can consider crit damage bonus on the crown for higher damage when you use the elemental burst.
For the weapons, because her elemental skill has a fairly long cooldown, it might be better to use Sacrificial Greatsword. This can let you deal additional damage, as well as more energy for your team. Serpent Spine can work well too since she can provide a shield, then you won't lose your stacks when you get hit. Blackcliff Slasher if you have her at constellations level 2.
If you are using Sacrificial Greatsword on her, then Pyro artifact sets like Lavawalker, or Crimson Witch of Flames can be an option to consider. This is because her Pyro attacks starts to become more reliable due to the reset, and energy gain so she can reach her burst faster.
Every 4s a character is on the field, they will deal 6% more DMG and take 3% more DMG. This effect has a maximum of 5 stacks and will not be reset if the character leaves the field, but will be reduced by 1 stack when the character takes DMG.
After defeating an enemy, ATK is increased by 12% for 30s. This effect has a maximum of 3 stacks, and the duration of each stack is independent of the others.
Since Retracing Bolide don't actually give the bonus effect to the character that you switch to, her main option for artifact set is most likely just Noblesse Oblige, a set that works on most support characters. If you only want her to be tanky, then you can consider Defenders Will so she can survive longer after taking strong hits. Be sure to go for DEF main stat and sub stats for your artifacts to increase her shield strength.
If you have her at constellation 2, she works quite well as a support DPS. Noblesse set is good for a lot of characters in general, and if you have been farming it a lot, chances are, you can build Xinyan. This is because having 2 piece Bloodstained and 2 piece Noblesse boosts the damage of her elemental burst by a lot. Equip her with a sacrificial greatsword and you can essentially bring the shield up a total of 3 times (2 from elemental skill, and 1 from elemental burst), as well as fast recharge for her high damage dealing burst.
Best support weapon of choice is definitely the Sacrificial Greatsword. For the same reasons as the DPS style. Being able to use your elemental skill again is a huge boost to your damage and team effectiveness by getting more energy.
Increases Elemental RES by 30% for each element present in the party.
Her constant Pyro application really helps any team that requires Pyro for some elemental reaction. Hydro damage is boosted by x2 if you attack an enemy with a Pyro, but it's not fast enough to be good with Tartaglia. You can consider Xingqiu if you want to boost his damage instead. If you want to boost Xinyan's damage, then bring someone that can apply Cryo often, like Kaeya, Diona, or Chongyun.
Xinyan always comes to the Wanmin Restaurant for a late-night snack! She gobbles up all the new dishes that the likes of Xingqiu and Chongyun don't go near... and nine out of ten times, she enjoys them too! She's great.