How can I modify cart amounts and inject UI elements with a custom app?

[AdityaSingh623]

I have a few queries regarding a custom Shopify app I am developing:

1. Is it possible to inject a UI element into a customer's storefront using custom Shopify app?
2. Can the order amount in the cart be updated when a customer selects to add loyalty points?
3. Is it feasible to fire a webhook and make an external API call when a customer logs in to collect their loyalty point data?

I would greatly appreciate it if you could provide any relevant documentation or resources that can assist me in implementing these functionalities.

Thank you for your assistance.

Best regards