If you are looking for the Old English aesc or ash on the Macintosh, it is created by using both the Option and the apostrophe key.
Such a helpful letter in OE.
In Microsoft Word, ash is made by CTRL + SHIFT + & followed by A.
Have fun spelling encyclopædia the way I grew up with it!
Note: It is also how mediæval, archæology, and some other word I can’t remember right now used to be spelled–which is why I had a hard time with those growing up. I had seen them in older texts and did not know how to spell them in “modern” English. (At the time I did not know that was the issue.)