Why split before the limit
The hard limits are 50,000 URLs and 50 MB per file, but smaller files — 10,000 URLs is common — make Search Console’s per-sitemap indexing reports far more useful: you can see which section of the site has coverage problems. Group by section when you can, then let the index tie them together.
Image, video, news and hreflang entries are carried through unchanged when files are split or merged.
Questions
Is anything uploaded?
No. Files are read and rebuilt in your browser; the ZIP is created locally.
Which lastmod is kept when a URL appears twice?
The most recent one.
Can I split a .xml.gz file?
Yes, gzipped sitemaps are decompressed in the browser.
How do I upload the result?
Put every file in the same folder on your site (usually the root), then submit only the index file in Search Console.