How to export photos in the same quality

That's right. Compression level ("quality") aside, image content plays a major part. The way the jpeg compression algorithm works, flat smooth areas compress much better than busy high frequency detail.

Here are two jpegs at 1080 x 720 pixels, both saved at the same compression level. The first is 67 kB, the second is 753 kB. The only difference between them is the content of the image:

This has a rather paradoxical side effect. The second time you save a jpeg, it will often be slightly bigger than the first time. This is because compression artifacts are read as added detail, and so less efficient compression.

When a jpeg is opened, it is decompressed back to its native size. An open file doesn't have a file format at all. File formats are storage containers, with the data packaged in a certain way according to the format spec. When you resave a jpeg, it once again goes from full size to packaged state, and the compression runs once again from scratch. It doesn't "remember" the previous compression.

In addition to what @D Fosse has said —

File size of JPGs does not indicate quality. You will make a lot of mistakes and drive yourself crazy by equating JPG file size to JPG quality.

A jpeg will never come out the same quality as it came in. Don't resave jpegs if you can avoid it.

Jpeg compression is destructive, cumulative and non-reversible, at any quality setting. Always, no exception.

The reason jpeg still survives as a file format, is that the compression is astonishingly effective at reducing storage and transfer size. Jpeg compression can shrink a file down to 2-5% of full native size, with acceptable visual quality loss. The emphasis is on "acceptable", there will always be some.

You should always treat jpeg as an end product for final delivery. It will usually work well for that. But not as a working/archive format.