he jump to the timeline back before 1970.
stays there until old. , until all time has past.
waaaayyyyy further which nobody knows how long.
old and retired, put on his 'old' quantum realm suits. jump to his 'past'. by doing this, he merged back to ENDGAME timeline, a bit earlier like few minutes earlier than he step oj that platform, behind the tree. take off his 'suit' and stays there. he MUST return, but he didnt. which explains he travel back to his 'past' from the 'old timeline'.

quantum realm machine is a portal.
you jump into it, you can choose whichever timeline "in the past" , and must return on that platform to stay in the same timeline.

if cap re-appear to that machine, he came back from any point of "the past" to the present time.

if he didnt, that means he never return.

but he can re-appear on another location of this timeline "only if" that time was his past. in this case:

eg-
2023 is young cap present (T1)
1940 is young cap past (T2)
2099 if he stays this long and grow old, he can merge with (T1) on any location before he jump into the machine, which became his past. he chose 'the bench' to make it dramatic.

proof: cap+tony NY jump to 1970 without the machine. , doesnt matter if he havent been there as long they know the coordinate (tony's tech GPS)

get it?