what will happen if/when another galaxy collides with ours
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“Collides” is a misnomer, galaxies are extremely empty, all considered. They will fly though each other, but gravity makes their objects interact.
Galaxies are often flat(ish) disks of stars and their surrounding stuff orbiting around a common center. A collision won’t do much to most individual star systems, as encountering another star very close (say, within one light year) is pretty unlikely. But the merger will throw many stars off their path around the galactic center. What then happens depends a lot on the details. The star might end up in the other galaxy, or fly off, or even end up in its initial galaxy but somewhere else.
On a large frame, the galaxies might pass through each other (and exchange stars and stuff), or they might merge into a larger galaxy. That new galaxy might just be an ovoid blob, or a smaller one might become a kind of ring around another ([depiction of how that looks for our galaxy right now](https://qph.cf2.quoracdn.net/main-qimg-ba258d067778267179b55ea381263508-lq)), they might create a [wheel](https://upload.wikimedia.org/wikipedia/commons/f/f0/Cartwheel_Galaxy_JWST_NIRCam%2BMIRI_Full_Res.png), or [many](https://cdn.sci.news/images/enlarge7/image_8308_1e-TXS-2116-077.jpg) [other](https://www.nao.ac.jp/en/contents/news/science/2019/20190618-alma-fig.jpg) [options](https://upload.wikimedia.org/wikipedia/commons/thumb/f/fc/Cosmic_%E2%80%9Cflying_V%E2%80%9D_of_merging_galaxies.tif/lossy-page1-946px-Cosmic_%E2%80%9Cflying_V%E2%80%9D_of_merging_galaxies.tif.jpg).
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