When you paste data into excel, or open a csv file in Excel it “helpfully” does an automatic data conversion on data that it deems needs it.
This can be great, it that is what you want. But it is very annoying if it isn’t! It even changes what you type into a cell, for example typing 021, will be automatically converted to 21. And Jan1 will be changed into a date (which are actually stored as numbers in Excel). Again, great if that is what you want to happen, not so good if you don’t.
You can now (this feature came widely available in 2024) easily control which data conversions happen.
Go to the File menu and choose options, (it is right at the bottom), then choose data (from the menu on the left), and the automatic data conversion section is at the bottom. Turn off any that you don’t want to happen.
You can hover over the information i to get some more detail on each of the options and that also contains a link to even more detail. It is worth checking out, as there may be some unintended consequences that you are not used to if you change some of these options.