In England the government have designated construction as an essential industry so there is no restriction on them getting back to work as long as they can fulfill the social distancing requirements.
The reason why many firms shut down at the start was because they could not fulfill the requirements and needed time to work out how they could continue.So they are going back with much altered working methods and revised production plans etc.
These are the guidelines that have been issued,
https://www.constructionleadershipco...n-3-published/
However Health and Safety law is still important and if an employer cannot maintain a safe place of work even with taking as many mitigation measures as are practical then they will be failing in their duty under the Health and Safety at Work Act and at risk.
Now some countries around the world don't have such legislation so it is not going to be an issue. In the UK it will be as that is the safety net for every one. Breaching health and safety law is subject to exemplary fines not least because of some severe failures in the past by, yes the construction industry.
These operating procedures are well worth reading to see how they might be adapted in a fight training environment where working in close proximity of others is a similar issue.