The link provided by
@maximus is great !
As for mods, the best place is
www.nexusmods.com.
You need to register but a free membership (with limited bandwidth) is fair enough for downloading SDV mods.
Download and install Vortex (a mod manager by nexusmods) making it easier.
You need to install "tool mods" in order to have more "real mods" working properly. They do include :
SMAPI
Content Patcher
Json Assets (if you want to use custom objects, there are plenty of good ones)
Spacecore (same)
Nexusmods warns you of pre-requisite mods when you download a new one, making it easy to first install these.
I'd recommend :
Eye Candy ([CP] means using the Content Patcher)
[CP] A Toned Down Stardew Valley (desaturated colors)
You might also like
[CP] Eemie's Map Recolors
[CP] Flower Valley
[CP] Seasonal Fences
[CP] Seasonal Floorings
[CP] Vintage Interface V2
[CP] Hojichas' Walls and Floors
[CP] Mi's and Magimatica Country Furniture
[CP] Industrial Kitchen and Interior
[CP] Industrial Furniture Set
[CP] v1.4 misc recolour
[CP]Yellog's wood craftables
Quality of Life
SkipIntro (while testing mods...)
UI Info Suite (life changer !)
LookupAnything (a kind of Encyclopedia allowing to "F1" any object hovered by the mouse pointer)
Cheats, more or less (I allowed myself to use them once I was rich enough and had completed the Grand'Pa's challenge of a 4-candles farm)
SkullCavernElevator
WinterGrass
BetterJunimos
GiftTasteHelper
Hint : you can reduce your silos to one by manually managing the hay between a chest and the only silo (pick it in the feeder inside, drop it on the silo from outside).
Cheers !