I am using Sitecore 10 with JSS - Angular App, I have a contact-us form in page and I need to submit the data of form and save it to Sitecore tree.
What is the best solution to do this?
Create API on top of Sitecore based on WebAPI and create a method that would process your call. Probably work for Sitecore Backend developer. You would just send request with all form params to this method and then save them in Sitecore. Don't forget about encoding/decoding form field values so nobody uses this gate to inject malformed and protentional risky content.
Another option would be to use Sitecore Forms -> https://jss.sitecore.com/docs/techniques/forms.