What platform are you on?
Which containers are safe?
The only containers that respawn are flora (flowers, shrooms, etc) and guild supply chests. A good starter home for most players is Hlaalo Manor, and at least one occupant (Ralen Hlaalo) will hold onto your belongings for safe keeping. If you're on PC I would recommend downloading a house mod or create your own with the CS.
Also, do things left on the ground disappear?
I've read lots of topics about it but it is still not clear for me
I've never had anything placed on the ground vanish, but I have experienced the "overflow loot bag" once. Here is an explanation from the UESP (not vouching for accuracy);
If you drop enough items in an interior cell (but not in a chest or other container), the next time you enter, a cloth sack will have spawned labeled "Overflow Loot Bag". If you check the bag, some of the items you dropped will be in the bag. After this point, any items dropped on the floor will go into the bag, but you can't put anything into the bag directly (only by dropping it on the ground). While this can be done in both the Xbox and PC versions, it takes 1024 items for it to happen on PC, compared to only 256 on Xbox. There's a way to get around this, however. When you drop the items, rest. Then leave. Come back. The Overflow Loot Bag will be there, but with nothing in it.
On the PC, Ctrl-Click will grab one item from a stack. To create the Overflow Loot Bag in a specific place, take a stack of items to the place you would like to create a loot bag, and drop them individually. When enough are dropped, you should have a loot bag, possibly with a few items in it. You may have to leave the cell and return.
Anything you drop on the floor in this location will go in the loot bag, which has an unlimited capacity.
You cannot put anything in the bag, but you can drop it on the floor, and the game will move it to the bag.
You can take items out of the bag normally.
You can have only one loot bag per cell.
It is a bad idea to try to create loot bags with small items. The loot bag will be created on TOP of the 256th item. If the 256th item is a small item that you need, you now can't get it, ever, because you can't move the loot bag, and you can't move your pointer to get the item, and the item will never move into the bag (the game only moves NEW items over the 256 limit into the bag). This may be overcome by using the "disable" command on the bag to effectively delete it, or possibly by using "tcl" to get a better angle so that you may pick up the hidden item.
When you drop an item on the floor so it goes into the bag, you have to leave the cell and return to see it actually in the bag. ***The GotY XBox version fixes the loss of the 256th item, just like any other item after it will be placed into the loot bag though unlike the other items you MAY HAVE TO WAIT 24hours OUTSIDE of the cell for it to appear.
If you create about a half dozen loot bags (Caldera is a good example), you'll have places to put any items without worrying about how much is there. If you have the GotY version, or the Tribunal patch, placing loot bags in High Fane, Mournhold, or Sotha Sil (when you receive the Mazed Band at the end), can be helpful to quickly get/drop items from anywhere.
Of course, if you wish to redecorate your house, the loot bag is a problem, but there is a solution of sorts. For Xbox this process takes some time. For PC it should be fast and simple, but either way it's well worth beating the Overflow Loot Bag. First, arrange your stuff the way that you want it in your house, and do NOT leave the house(or vicinity if you are doing it outside). Next, save and then reload that saved game. All your items should stay where you left them, even if you leave your house. Note that placing NEW items after this will be sent into the loot bag.