By default, locolamp configures Apache to use the ./web folder (literally $LOCO_PRJ/web) as the web-root. What if your web-root lives somewhere else, such as $HOME/src/webapp? Simply edit .loco/loco.yml and set the variable HTTPD_ROOT=$HOME/src/webapp.
(Similarly, if you need to make a more nuanced changes to the configuration, edit the template .loco/config/apache/conf/httpd.conf.loco.tpl.)
These changes will not necessarily take effect on their own. You may need to reinitialize the Apache service before starting. This will overwrite/destroy any auto-generated state/configuration:
[nix-shell]$ loco init apache -f -v
[nix-shell]$ loco run
Since the services are only used by the local development project, I often don't care about retaining the data. It's easier to destroy+reinitialize+run all services at the same time, which can be boiled down to one command:
[nix-shell]$ loco run -f -v
See also: Quick start with alternate HTTP web-root