A few questions regarding building as app using shopify storefront

[TheJosh]

We are building a shop using storefront. The frontend will be created using Next.js and I will be using the GraphQL storefront to communicate with shopify.

On the pricing page I have a few questions.
Q1. Shopify says that we will get a free SSL certificate. Does this mean I will not need to buy an SSL certificate for my website? I will be purchasing a custom domain.

Q2. In the developer account, I cannot add a custom domin. It says that my plan does not cover this. Which plan do I need to enable this feature? Or is it available in all plans in the production/paid account?

Q3. "Third-party calculated shipping rates. Show calculated rates with your own account or third-party apps at checkout." Will I be able to add custom shipping rates for my products? In the settings page in the dev account, I have access https://admin.shopify.com/store/<app>/settings/shipping/profiles/<some_id> Will this be disabled if I use the basic plan?

Q4. What is shopify synced orders? What is meant by "First 50 Shopify-synced orders"

Q5. Can I integrate any payment gateway that I want? Shopify says the "Transaction fees" is 2,1 and 0.5% for the plans. I'm building the shopify app using the GraphQL Storefront, will the payment gateway be handled by shopify? Or can I integrate my own choice for the gateway. Will the Transaction fee for shopify still apply in that case?