I'm personally curious as to the impact of Senator Mike Mansfield's (D) amendment to the fiscal year 1970 Military Authorization Act (Public Law 91-121, Section 203) on science research and funding thereafter. The amendment prohibits the DoD from funding research not directly applicable to military functions. I'm reading a variety of opinions and documents from that time, but I'm stuck on a more basic question: How does this all work in practice? Specifically, what does it mean to amend a bill? Can a senator introduce an amendment to a bill on a whim? Does a Y vote on a bill mean a Y on the amendments put forth by congress members? This bill outlines DoD spending in 1970. But isn't DoD spending re-evaluated every year? If so, does the amendment still apply years after the fact? I mean, it's seemingly impossible to keep track of every little amendment passed over time, so I can't imagine the government in 2023 reminding itself to comply with budget restrictions passed 50 years ago, especially since (I'm assuming) budgeting is always changing.
While a bill is being considered in Congress, any member may propose amendments. There are several opportunities for doing so, either in committee, or on the floor of the House/Senate. There are particular procedures to go through - so amendments can't be done "on a whim". Each amendment is considered (by the Chamber, or by the committee) and voted on. If the motion passes then the amendment becomes part of the bill.
Finally, after all amendments have been considered each chamber gets to vote on the final text of the bill and after it has been signed by the President it becomes Law. The bill, as presented to the president, and as published, is the amended bill. Lawyers don't need to keep track of amendments. Indeed in the bill as published there is no record of what was an amendment and what was in the first draft.
And yes! It remains law until either it is repealed, or replaced by a new law.
The Law states, therefore, that funds appropriated under Public Law 91-121 may only be used to fund research that has a direct and apparent relevance to military function. However since the act only authorises appropriation in the fiscal year 1970, this law has no direct effect today.
However, there are military appropriation acts each year, so this clause may have set an informal precedent, and each subsequent bill has such a clause. That's not binding, of course.
You don't need to "keep track of every amendment" You only need to keep track of the Act's final text. And knowing what the law is, is the job of lawyers. There's no particular difficulty in keeping track of the Law.