I guess this is particular to my setup, but since I'm using vercel on the front end for nextjs I just do it on their website. This is great if your website is all static.

server side I usually just use an ec2 instance. On there I usually use systemd unit file to run my server. In the unit file I put all my env. variables. But in code I just use dotenv github.com/motdotla/dotenv