Hi! As you can guess, your custom Apache config will take a little work to migrate to Nginx but it is 100% doable. Here’s an older tutorial with some good info to get started:

https://www.digitalocean.com/community/tutorials/how-to-migrate-from-an-apache-web-server-to-nginx-on-an-ubuntu-vps

Pay attention to the comments as there is some good info in there too, like this online converter for some of the rewrite rules:

https://winginx.com/en/htaccess

Hope this helps and if you get stuck please follow up in here and we’ll help you work through any issues if possible.

Good luck!

Thank you!
I read the guide and this is how far I have got:

server {
    root /var/www/example.com;
    index index.php;

    server_name example.com www.example.com;

    include global/restrictions.conf;

    location / {
        try_files $uri $uri/ /index.php?$args;
    }
    location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
        expires max;
        log_not_found off;
    }

    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_intercept_errors on;
        fastcgi_pass unix:/var/run/php/php7.3-fpm.sock;
    }

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}

server {
    if ($host = www.example.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    if ($host = example.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    listen 80;
    listen [::]:80;

    server_name example.com www.example.com;
    return 404; # managed by Certbot
}

And the restrictions.conf file:

# Global restrictions configuration file.
# Designed to be included in any server {} block.
location = /favicon.ico {
    log_not_found off;
    access_log off;
}

location = /robots.txt {
    allow all;
    log_not_found off;
    access_log off;
}

# Deny all attempts to access hidden files such as .htaccess, .htpasswd, .DS_Store (Mac).
# Keep logging the requests to parse later (or to pass to firewall utilities such as fail2ban)
location ~ /\. {
    deny all;
}

# Deny access to any files with a .php extension in the uploads directory
# Works in sub-directory installs and also in multisite network
# Keep logging the requests to parse later (or to pass to firewall utilities such as fail2ban)
location ~* /(?:uploads|files)/.*\.php$ {
    deny all;
}

This works for the main site, I can now navigate my main single Wordpress install which is in the root folder.
However the Wordpress Multisite install which is located in folder “/u” is still broken.

I think I solved it.

This is what I got:

server {
    root /var/www/example.com;
    index index.php;

    server_name example.com www.example.com;

    include global/restrictions.conf;

    if (!-e $request_filename) {
        rewrite /wp-admin$ $scheme://$host$uri/ permanent;
        rewrite ^/u(/[^/]+)?(/wp-.*) /u$2 last;
        rewrite ^/u(/[^/]+)?(/.*\.php)$ /u$2 last;
    }
    location / {
        try_files $uri $uri/ /index.php?$args;
    }
    location /u/ {
        try_files $uri $uri/ /u/index.php?$args ;
    }
    location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
        expires max;
        log_not_found off;
    }
    location ~* ^.+\.(ogg|ogv|svg|svgz|eot|otf|woff|mp4|ttf|rss|atom|jpg|jpeg|gif|png|ico|zip|tgz|gz|rar|bz2|doc|xls|exe|ppt|tar$
        access_log off;
        log_not_found off;
        expires max;
    }
    location ~ \.php$ {
        include snippets/fastcgi-php.conf;
        fastcgi_intercept_errors on;
        fastcgi_pass unix:/var/run/php/php7.3-fpm.sock;
    }

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/example.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/example.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}

server {
    if ($host = www.example.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    if ($host = example.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot


    listen 80;
    listen [::]:80;

    server_name example.com www.example.com;
    return 404; # managed by Certbot
}

Am I missing anything important?

if (!-e $request_filename) {
rewrite /wp-admin$ $scheme://$host$uri/ permanent;
rewrite ^/u(/[^/]+)?(/wp-.) /u$2 last;
rewrite ^/u(/[^/]+)?(/..php)$ /u$2 last;
}

is that part necessary at all?