the stem

the whole plant
I know I should repot it to a bigger pot.
maybe this week I will

Looks like Leaf Miners were in the leaf. As the linked article indicates the adults that lay the eggs that become the larva that make those tunnels are becoming immune to most of the available insecticides, which also kill off any predators there might be.
I'm not sure what might be available in Lebanon.

Here is a link that might be useful: about leaf miners

and the bottom of the stem? is it bcuz it needs repotting? or bcuz of the leaf miners?

ID the plant.. then you can figure out if the stem is normal or not ...

name that plant forum if need be ...

looks overly mature to me ... i would learn how to propagate it based on the ID ... and once i had some new ones... i would get rid of grandpa ... it is allowed in the plant world ...

the houseplant forum may also help if it is one ...

ken

Does that pot have a drainhole?
If not, that may be the problem with the stem.

plant id: kalanchoe
and the pot do have 3 drain holes in the bottom

I just remove leaves that look like that and throw them in the trash so they don't spread to another leaf. I don't think there is anything you can spray that would penetrate the leaf.

The leaf miner that made those trails in that leaf will not move to another leaf, although it will drop to the soil to emerge someday as an adult saw fly, probably. The trails left are more cosmetic than harmful, so there is no real need to remove and trash any affected leaf. There is nothing, no poison, that will reach the larva inside the leaf. However, if you see some movement inside that leaf then the larva is still there and squishing it is quite an effective method of control.
Perhaps some time spent at the linked web site would be beneficial.

Here is a link that might be useful: About leaf miners

I did just cut the affected leaves yesterday (they were few). I put light behind each leave and spotted them

That was a good idea! Hopefully that is the end of them.