PG index architecture

I was thinking about indexes, and am kinda curious about sequential access.

I know nothing of PG guts, so this might even be a dumb question.

As I understand indexes, they are a key value pair, that contain a value
and a position. You lookup the value then use the position to seek into
the database to load the record.

Do we, or could we, load all the the matching index records, then sort
them by position? (maybe not all, maybe large batches)

When loading from the database, if access was slightly more sequential
(vs very random), would it increase performance?

Said another way:

I think of table scanning as sequential, and fast. That would be
loading db record 1,2,3, etc.

Would it be faster to load db records "mostly sequential": 1,3,4,7,10
compared to randomly: 7,3,10,1,4

-Andy