Understand the starting family
Standard LaTeX classes traditionally use Computer Modern. A template or package may select something else, so inspect the preamble before assuming a default.
Overleaf documents package-based alternatives such as TeX Gyre Bonum. The family selection is separate from choosing a 10pt or 12pt document base.
Use the workflow supported by the engine
For a traditional package-based setup, choose a documented family package compatible with the template. For XeLaTeX or LuaLaTeX, fontspec can select available modern fonts with commands such as \setmainfont{...}.
Do not add fontspec to a pdfLaTeX project and expect the same setup to work unchanged. Use the compiler required by the template or deliberately adapt it.
Set roles and confirm resources
With fontspec, main, sans and monospaced roles can be specified separately using \setmainfont, \setsansfont and \setmonofont. The named font must actually be available in the build environment.
A font installed on your personal computer is not automatically installed on a cloud compiler. Keep the source files and their permitted use with the project when supplying a custom resource.
Treat math as a separate system
A text-family change does not necessarily choose a matching mathematical font. Read math font commands before changing notation, and font file types when comparing resources.
Your questions, answered.
Will a font installed on my computer automatically exist in Overleaf?
No. The cloud environment has its own resources; custom files require an appropriate project setup.
Does setmainfont automatically change all math symbols?
Do not assume so. Text and mathematical font setup are separate concerns.
Sources & background
Sources supporting this page’s explanations. Practical examples are our editorial suggestions.