What happens when you enter http://[site] in a browser and press Enter? Why does a HTTPS lock sometimes appear, and sometimes an error?

When you enter a URL in the browser and press Enter, several steps occur:

1. Parsing the URL and determining the protocol (http or https).
2. Finding the website's IP address via DNS.
3. Establishing a TCP connection with the server.
4. If the protocol is HTTPS, a TLS handshake occurs for encryption.
5. Sending an HTTP request to the server.
6. Receiving the response and displaying the page.

The HTTPS lock appears if the connection is secured with a TLS certificate, which:

• Is valid (not expired).
• Issued by a trusted certification authority.
• Matches the website's domain.

If the certificate is missing, expired, or invalid, the browser will show a security error, warning the user of the risks.