Basically, Shirou, due to having Avalon, and it being active sense it was implanted in him, was able to summon every servant related to Camelot, including their other forms, Such as Saber Lily, Artoria Lancer, and Goddess Artoria.

Avalon made the Command seals evolve  into a state where they recharge over an hour, and that when he no longer has any, they will still recharge, Avalon has also given Shirou a High quality Fae Magical core due to the summoning, his Projections can now be made real, and grow beyond the original, with the power of fae runes being weaved in during Alteration and Reinforcement.

Artoria remembers her time as Shirou's servant in other timelines, as well as her time as Saber Alter. So when she appears, she immediately lounges at him, being filled with pain, relief, guilt, and joy, to remember him and be with him again.

Merlin also used his as a loophole to summon himself in a similar way to how he does in FGO. Also, Vivian is here!

Lancelot is Summoned as Saber with his memorys of his berserker self from the previous war.

Shirou's projection has now evolved to basically Blade Blacksmith and Sword birth from Highschool DxD, and can combine the Trace of multiple weapons and project and new weapon made of proper metals due to a few nudges in the right direction by Merlin, and Vivian, cus she finds his Magecraft fascinating due to his limitation from being a incarnation, and how his Origin and Element have now evolved from Avalon's influence.