Which SSD to buy for Inspiron 3593

If you use a SATA 2.5" SSD, you will need to replace the existing hard drive.

You can add the M.2 drive (it must be NVMe, as the system does not support a SATA M.2 drive).  Before buying, be sure the mounting bracket and screw is present in the system for the drive -- if it's not, you'll need to order that from Dell or a Dell parts seller like parts-people.com.

There are many manufacturers of good SSDs -- WD, Samsung, Crucial and others.  

How do I know if the mounting bracket for M.2 NVMe is present or not? I won't open it myself.. I was planning on taking it to the repair shop. Also does this system not support SATA M.2? I bought this laptop in 2019 and why does it support the newer NVMe and not SATA M.2?

Btw should I get a 2.5" SSD or a M.2 NVMe? Is the performance different and considerable or I can get either? Which one would you suggest for widows 11, netflix/youtube and other light activities and perhaps rarely light gaming?

Storage options quoted below from the technical documentation for the system.  No, this system does not support SATA - it is NVMe only.  And any M.2 NVMe drive will be an order of magnitude if not more, faster than a 2.5" SATA or M.2 SATA drive.  

As for the rest, if you're going to have the shop do the work, let them deal with the compatibility and mounting hardware issues.  There's no way to know what's inside without opening the system

So should I buy a M.2 NVMe SSD online and give them or let them open the system? Either way it is complicated.. cuz I don't know what they have in store and my understanding is that buying online is less expensive.

Mounting hardware should not be a problem, as long as you go with a 2280 M.2 SSD. A 2230 M.2 will be shorter in length, and thus a mounting bracket will be required in order to extend over to the screw mount.

If you are going to have a repair shop do the work, they should have an appropriate mounting/hold down screw on hand. Otherwise, it maybe already present inside the computer, much like it was in my Inspiron 3583.

As stated before, you will need to install a NVMe M.2 SSD. NVMe's transfer speeds are faster than the SATA interface, so you this will be a big benefit. It won't hurt to leave the existing hard drive in the system, which can later be re-purposed for additional storage, like for storing personal files, music, videos, photos, etc. Just make sure that the system boots from your new NVMe SSD before re-formatting the HDD.

Any SSD brand will do, but if you want to ensure reliability, buy a good name brand SSD, like Crucial or Samsung, and not the cheapest one that you can find. Also, look for one with a DRAM cache to further help improve performance, especially if you transfer large amounts of data.