Merging sheets

The Album feature that existed in Mu2 had been removed for Mu3 and never came back, regardless there having been a GSoC project to get it back to Mu4
There's a PR against my MuseScore 3.7 Evolution though, that may work for you here, see https://github.com/Jojo-Schmitz/MuseScore/pull/79

"I can't find how to combine 2 different sheets"

The "quick and dirty" method is to have two or more sheets exported from MuseScore as PDF files. Then you can merge the PDF files with any suitable free PDF tool, for example:
PDFsam Basic: https://pdfsam.org/download-pdfsam-basic/ (download the app)
Smallpdf: https://smallpdf.com/merge-pdf (use online)

I'm so much glad to know more about it. Keep up the good work.

I have been waiting to acquire a new Mac in order to be able to use MuseScore 4. I have now got the machine, but I have been very disappointed to find that the album facility has not (yet?) been incorporated in version 4. I regularly produce dance-set sheet music for Scottish dancing, which typically involves combining four or so tunes. Each tune is held as a separate score (because tunes can be combined one way and then recombined another). To combine them in a single MuseScore score I have to copy and paste from the single tune scores to the combined one. There are various snags in practice. Copy and paste deals only with the notation (notes, chord symbols) but not with the key and time signatures or with the repeat lines and barlines. One has to be careful with partial bars to set up an empty bar with the right number of beats in order to copy correctly. Heading/title frames have to be separately edited. One can get quite fast at it, but it was much smoother with the album feature!