email templates - how customizable are they?

were moving our transactional emails in-house (order confirmations, shipping updates, etc). couple questions: - can we use our own branding (colors, logo, fonts)? - are there dynamic fields like customer_name, order_number etc? - can we preview how it looks on mobile vs desktop? - is there a way to test deliverability before sending to real customers? we were using sendgrid dynamic templates before but the editing experience was painful. literally editing json to change a heading