Does anyone know why some of my .package files in my Downloads folder modify themselves? Today I saw a mod that was made in 2009 and that was modified in june 2020, whereas I never touched it, and the file size (.package) is bigger now. I don't know if it's normal and it's the game that does that?

Files shouldn't get modified on their own, but there are a few cases where they could appear to be larger/smaller or modified.

Have you used the Compressorizer or opened the files with any program like SimPE, Delphi's organizer, the Wardrobe Wrangler, or any of the sort?
Have you at some point downloaded new version of these files?
Are you looking at the "date" or the "date modified"? - these can sometimes be different.

Is your anti-virus set to ignore Sims 2 files? Make sure it is.

My computer considers a file "modified" when I take it out of a zip folder, or move it in and out of a folder that's used by any cloud service (probably among other innocuous situations I'm simply not aware of.)

How did you realize it was bigger, and by how much?
And when did you download said mod?

If the file contains a sim, like a custom NPC for example or even a partial override of a default NPC it will get modified and saved by the game.

I've had that happen because I updated to a new version of that Mod though it can happen if there's an NPC included with the hack or it replaces NPCs.The only other time was a glitchy virus program known as AVG the last time I had that installed and it was a horrible mess back then though I don't know if it ever got fixed since then because I got too scared to reinstall it after it started removing what it though were dulpicate files which were actually back up copies I made on purpose.

Hi, I didn't open the file in any program. The mod in question is ""Select your cemetery" after sims died": http://modthesims.info/d/316426
I just downloaded it yesterday and after tested it in the game in a test neighborhood to see if the mod was correctly working, I saw that the Date Modified of the file was updated to today.

I don't think so, my antivirus is Windows Defender but I'll check it out

Yesterday, I downloaded this mod: http://modthesims.info/d/316426 ("Select your cemetery" after sims died)
I tested it in a test neigborhood to see if it was working and after my test I checked my Downloads folder if any file was modified in the last month and I saw the mod was modified today. (I check recently now because that's not the first time that is happening...) All my other mods say the real date of the upload like "2009", and when I redownload the mod and place it in my Downloads folder, it's say the real date (2009). The file went from 59ko to ~80ko if I remember correctly.

That is a partial override of Grim Reaper so it is not uncommon for the game to alter it, probably added a thumbnail image or something to it.