How to split up compose file into multiple yaml files

i currently have a docker-compose.yaml file with a lot of services that depend on each other.the volumes, secrets, and networks tags take a lot of lines. so i thought i would export them into multiple files.

here is what i did:

# volumes.yamlx-volumes: &volumes  driver: local  driver_opts:    type: bind    o: bind    device: /data/${{str}}vol1:<<: *volumes  str: bla1vol2:<<: *volumes  str: bla2

and for the networks:

# networks.yamlnginx:DLNA:  driver: macvlan  driver_opts:    parent: eth0  ipam:    config:      - subnet: 192.168.178.0/24        gateway: 192.168.178.1        ip_range: 192.168.178.250/32vpn_apps:  driver: bridge  driver_opts:    com.docker.network.bridge.name: "docker-apps"    com.docker.network.bridge.enable_ip_masquerade: "true"    com.docker.network.bridge.enable_icc: "true"    com.docker.network.driver.mtu: "1500"  ipam:    driver: default    config:      - subnet: 172.21.0.0/16        gateway: 172.21.0.1

and for secrets i did something similiar:

# secrets.yamlsec1:  file: /opt/secrets/sec1sec2:  file: /opt/secrets/sec2

then in my main.yaml:

version: "3.8"volumes:<<: ./volumes.yamlnetworks: <<: ./networks.yamlsecrets:<<: ./secrets.yamlservices:  portainer:    image: *images.portainer    container_name: portainer    restart: always    volumes:      - vol1: /some/path      - vol2: /some/path2    networks:      - nginx    secrets:       - sec1      - sec2

I'm getting the error: "yaml: map merge requires map or sequence of maps as the value"