Essentially what the supreme court ruled was Biden (or any other POTUS) cannot use executive orders to circumvent the Legislative Process and the Separation of Powers clauses of the US Constitution. Executive orders may be used to "temporarily" enact provisions in cases of NEW, EMERGENCY, AND OF A NATIONAL SECURITY NATURE until such time as the Legislative body may consider the matter under the legislative process. The SCOTUS ruled the Congress has not only had ample time to enact such a mandate should it have intent, it has indeed already addressed the issue on numerous occasions and resulting legislation never came close to deciding to include such mandates in the bills which were passed and presented as a result.
The SCOTUS ruled the limitations on the POTUS to use executive authority to enact provisions it cannot achieve through electoral process of The People is VERY LIMITED and narrow in scope. It is restricted to issues of extreme national importance of such an emergency nature and expressly of such broad approval by the electorate as to preclude them from being addressed by the provisions of the legislative process yet clearly to be of the will of The People and are further limited for a duration considered reasonable for the legislative body to address. Nor may such Executive writs be designed to instill a permanent solution where other measures of a less permanent nature could be put in place.
In other words, the POTUS cannot just dictate orders of any kind. The POTUS cannot expand the authority of OSHA beyond the scope of its operation. They ruled OSHA by Congresses Authorization may only address matters specific and unique to workplace safety. Clearly the dangers of COVID are not unique to workplace safety but apply to both non-workers and workers in their off hours. They must be narrowly tailored to address new and unprecedented situations. Such orders must be temporary and only put in place long enough for the matter to be addressed in accordance with the process defined in the Constitution, they must concern matters of national security of an emergency nature. Remedies must not be of a permanent or non-retractable nature when less permanent resolutions are available as options for the legislative body (e.g. Vaccine mandates cannot be undone but stay at home and temporary mask mandates can be undone). It must also be so clearly supported by the States and The People as to reasonably presume the decision would pass the scrutiny of the States and The People. Orders cannot be used to override decisions reserved to the States. It is assumed if the states have had time to address the issue and it truly is of such a level of emergency then the Congress would likewise have had opportunity to address. In this case they DID address it and apparently the majority opinion of the Congress did not feel such mandates were warranted. Overriding a State Statute indicates the Congress has had opportunity to address.