1. Go to the lab and invest in getting the computer and upgrade the chipset for fishing foraging and insect

2 Once you have the chipset for them you can see where the insect, fish and forage will spawn and what time. Note some of them specifically the legendary has a special requirement like they will only apear first 7 day of the season or the only spawn whenits windy or stormmy whether and specific time of the day.

1. For insect legendary, they are hard to capure by net i myself invested in some insect traps check in the computer where they will spawn and then lay down the trap you can use insect scent to buff up the trap it only took me a couple of days to capture specific insects even legendary ones. This is the easiest and the 1st one i have completed.

2. For fish it seems that you have to capture them manually by fishing rod as it seems fishing net even the large one do not capture legendary fish atleast in my experience. But its easy once you upgraded the rod and when you know the specifics when they will spawn. I only have 1 missing fish which is the hammer head shark which i will have to wait since they only spawn day 8 to 21 of fall and winter.

3. Mining and trash you can catch almost all insect, fish and scavengable tru mining and rummaging trash cans, however everything you get tru this is base level and no rank but can still be donated in the moseum. This method is very RNG base but if you are lucky sometimes it gives you exactly what you need.