How Do I Get Custom Content/Mods for The Sims 4

It's all really confusing (although I've been playing sims for a VERY long time) for a few reasons:

1) I do not know how to put mods and custom content on The Sims 4, or what 'files' it goes in (apparently you put things in files and I'm unsure of that)

2) I don't want my game to be laggy

3) I'm worried about installing mods and custom content online because it could contain viruses or make my game and computer laggy, so what websites are safe, or is there a way to make your own custom content and mods using 'Paint' or something every computer has already installed on it (there must have been a way mods and cc were made in the first place).

I really need help because cc and mods look great but I just don't know how to put it in my game. If you have custom content and mods and your game isn't laggy then please can you tell me what you did or if you know how to make your own custom content and mods (preferably using applications such as 'Paint' or 'Wordpad' or 'Microsoft Office Word, Powerpoint, Publisher, ect. that don't need downloading).

Thank you so much if you can help me out! 😉 <3 🤔