Section 51 would be a good place to start. Anything that is not specifically listed in S.51 remains a state, rather than federal, matter. You'll note that health is not listed in S.51. Lockdowns, travel and gathering restrictions, mask mandates and the like are being dealt with by the states as directions under their respective public health acts. The High Court has already upheld the states' constitutional rights in that regard.
In any event, how would Federal legislation overriding state-based restrictions, like border closures or mask mandates, work? Those matters are being enforced, by and large, by state police. Are you going to send in the Army to stop the various state police enforcing state legislation?
No I wasn't arguing that. The only thing they could do is attempt to get back control of the State Borders federally. That could help the airlines tremendously. I would agree that there is nothing the Feds can do about local State of Emergency Powers and enforcement it's a State matter.
From what I have read the Border issue is still a open question which hasn't really been contested and as usual 6 Lawyers have 8 opinions on the matter so it really depends on who you are asking. The current Federal Government hasn't really challenged the States over Border Control even though there are provisions in Section 117 & 92 that would suggest freedom of movement but these aren't absolute. Since noone is going to make a challenge I would suggest it will stand as is.
My original point is that really nothing can be done as all the States are running their own show hence the appearance that there is no leadership. As Lead Balloon suggests it may be a deliberate strategy.