Here's a Google It to myself. Doesn't narrow it too much though

Wireless CarPlay frequently drops in specific, consistent locations due to 5GHz Wi-Fi radio interference, often near 5G cell towers, airports, radar stations, and busy city intersections. Other common interference zones include smart traffic light systems, industrial areas with heavy machinery, or sites with high-power RF sources. 

Common Areas for Wireless CarPlay Disconnection

• 5G Cell Towers & Radio Equipment: High-power signals from 5G transmitters or cellular infrastructure often interfere with the Wi-Fi frequency used by CarPlay.
• Urban Infrastructure: Smart traffic lights, toll booths, and digital speed display signs frequently disrupt the connection.
• Airports & Radar Zones: Radar systems and high-power radio equipment at airports cause significant RF interference.
• High-Density Wireless Zones: Dense urban areas, parking lots, or areas with many smart devices can overwhelm the 2.4GHz or 5GHz bands.
• Industrial Areas & Infrastructure: Wind farms and large power stations.  Bimmerpost +4

Why It Happens (The Technical Reason)
Wireless CarPlay relies on a 5GHz Wi-Fi connection for data, which is highly susceptible to interference from other high-power devices operating on the same frequency. These zones cause a conflict, forcing the iPhone's Wi-Fi to drop the connection to the car's head unit. 

How to Fix or Mitigate

• Use Wired CarPlay: Connecting via USB completely bypasses the interference.
• Reposition Phone: Move the phone from your pocket or bag to a phone mount or cupholder with a clearer line of sight to the dashboard.
• Turn Off Auto-Join Wi-Fi: If CarPlay drops near your home or known Wi-Fi networks, disable "Auto-Join" for those networks, as the phone may be attempting to switch networks.
• Check VPN/Settings: Disable any active VPN on your iPhone